tests/functional: Output an actually correct command to accept test changes

I've run into this quite a few times when working with characterization test
infra. It would print an invalid command:

_NIX_TEST_ACCEPT=1 meson test main/lang

Which you'd then proceed to run and it would fail. This commit makes it
be honest about the command you need to run:

_NIX_TEST_ACCEPT=1 meson test --suite main lang

tests/functional/characterisation/framework.sh

@@ -71,7 +71,7 @@ function characterisationTestExit() {
             echo >&2 ''
             echo >&2 'You can rerun this test with:'
             echo >&2 ''
-            echo >&2 "    _NIX_TEST_ACCEPT=1 meson test ${TEST_NAME}"
+            echo >&2 "    _NIX_TEST_ACCEPT=1 meson test --suite ${TEST_SUITE_NAME} ${TEST_NAME}"
             echo >&2 ''
             echo >&2 'to regenerate the files containing the expected output,'
             echo >&2 'and then view the git diff to decide whether a change is'

tests/functional/common/test-root.sh

@@ -1,4 +1,5 @@
 # shellcheck shell=bash
 
-TEST_ROOT=$(realpath "${TMPDIR:-/tmp}/nix-test")/${TEST_NAME:-default/tests\/functional//}
+TEST_SUBDIR="${TEST_SUITE_NAME:-default}/${TEST_NAME:-tests/functional/}"
+TEST_ROOT=$(realpath "${TMPDIR:-/tmp}/nix-test")/"$TEST_SUBDIR"
 export TEST_ROOT

tests/functional/meson.build

@@ -263,7 +263,8 @@ foreach suite : suites
         'ASAN_OPTIONS' : asan_options,
         '_NIX_TEST_SOURCE_DIR' : meson.current_source_dir(),
         '_NIX_TEST_BUILD_DIR' : meson.current_build_dir(),
-        'TEST_NAME' : suite_name / name,
+        'TEST_SUITE_NAME' : suite_name,
+        'TEST_NAME' : name,
         'NIX_REMOTE' : '',
         'PS4' : '+(${BASH_SOURCE[0]-$0}:$LINENO) ',
       },