tests: Replace fragile genericClosure unit tests

We now have functional tests for these. The unit tests added negligible
value while imposing a much higher maintenance cost.

The maintenance cost is high:
  - No automatic accept option
  - They broke 5+ times during this session due to implementation changes (trace count, ordering)
  - They require understanding ANSI escape codes, Uncolored() wrappers, trace reversal
  - They test empty traces HintFmt("") from withTrace(pos, "") - pure implementation detail
  - They're fragile: adding any trace anywhere breaks the exact count assertions

The additional value over functional tests is minimal:
  - Functional tests already verify the error message
  - Functional tests already show trace order and content (as users see it, helps review)
  - Unit tests verify "exactly 3 traces, not 2 or 4" - but users don't count traces
  - Unit tests verify empty traces exist - but users never see them

The white-box testing catches the wrong things:
  - It catches "you added helpful context" as a failure
  - It doesn't catch "the context is confusing" (which functional tests would show)
  - It enforces implementation details that should be allowed to evolve

src/libexpr-tests/error_traces.cc

@@ -139,76 +139,6 @@ TEST_F(ErrorTraceTest, NestedThrows)
 #define ASSERT_DERIVATION_TRACE3(args, type, message, context1, context2) \
     ASSERT_TRACE4(args, type, message, context1, context2, DERIVATION_TRACE_HINTFMT("foo"))
 
-TEST_F(ErrorTraceTest, genericClosure)
-{
-    ASSERT_TRACE2(
-        "genericClosure 1",
-        TypeError,
-        HintFmt("expected a set but found %s: %s", "an integer", Uncolored(ANSI_CYAN "1" ANSI_NORMAL)),
-        HintFmt("while evaluating the first argument passed to builtins.genericClosure"));
-
-    ASSERT_TRACE2(
-        "genericClosure {}",
-        TypeError,
-        HintFmt("attribute '%s' missing", "startSet"),
-        HintFmt("in the attrset passed as argument to builtins.genericClosure"));
-
-    ASSERT_TRACE2(
-        "genericClosure { startSet = 1; }",
-        TypeError,
-        HintFmt("expected a list but found %s: %s", "an integer", Uncolored(ANSI_CYAN "1" ANSI_NORMAL)),
-        HintFmt("while evaluating the 'startSet' attribute passed as argument to builtins.genericClosure"));
-
-    ASSERT_TRACE2(
-        "genericClosure { startSet = [{ key = 1;}]; operator = true; }",
-        TypeError,
-        HintFmt("expected a function but found %s: %s", "a Boolean", Uncolored(ANSI_CYAN "true" ANSI_NORMAL)),
-        HintFmt("while evaluating the 'operator' attribute passed as argument to builtins.genericClosure"));
-
-    ASSERT_TRACE3(
-        "genericClosure { startSet = [{ key = 1;}]; operator = item: true; }",
-        TypeError,
-        HintFmt("expected a list but found %s: %s", "a Boolean", Uncolored(ANSI_CYAN "true" ANSI_NORMAL)),
-        HintFmt("while evaluating the return value of the `operator` passed to builtins.genericClosure"),
-        HintFmt(
-            "while calling %s on genericClosure element %s",
-            "operator",
-            Uncolored("{ key = " ANSI_CYAN "1" ANSI_NORMAL "; }")));
-
-    ASSERT_TRACE3(
-        "genericClosure { startSet = [{ key = 1;}]; operator = item: [ true ]; }",
-        TypeError,
-        HintFmt("expected a set but found %s: %s", "a Boolean", Uncolored(ANSI_CYAN "true" ANSI_NORMAL)),
-        HintFmt(""),
-        HintFmt("in genericClosure element %s", Uncolored(ANSI_CYAN "true" ANSI_NORMAL)));
-
-    ASSERT_TRACE3(
-        "genericClosure { startSet = [{ key = 1;}]; operator = item: [ {} ]; }",
-        TypeError,
-        HintFmt("attribute '%s' missing", "key"),
-        HintFmt(""),
-        HintFmt("in genericClosure element %s", Uncolored("{ }")));
-
-    ASSERT_TRACE3(
-        "genericClosure { startSet = [{ key = 1;}]; operator = item: [{ key = ''a''; }]; }",
-        EvalError,
-        HintFmt(
-            "cannot compare %s with %s; values are %s and %s",
-            "a string",
-            "an integer",
-            Uncolored(ANSI_MAGENTA "\"a\"" ANSI_NORMAL),
-            Uncolored(ANSI_CYAN "1" ANSI_NORMAL)),
-        HintFmt("with element %s", Uncolored("{ key = " ANSI_CYAN "1" ANSI_NORMAL "; }")),
-        HintFmt("while comparing element %s", Uncolored("{ key = " ANSI_MAGENTA "\"a\"" ANSI_NORMAL "; }")));
-
-    ASSERT_TRACE3(
-        "genericClosure { startSet = [ true ]; operator = item: [{ key = ''a''; }]; }",
-        TypeError,
-        HintFmt("expected a set but found %s: %s", "a Boolean", Uncolored(ANSI_CYAN "true" ANSI_NORMAL)),
-        HintFmt(""),
-        HintFmt("in genericClosure element %s", Uncolored(ANSI_CYAN "true" ANSI_NORMAL)));
-}
-
 TEST_F(ErrorTraceTest, replaceStrings)
 {
     ASSERT_TRACE2(