Move unit tests to the location Meson expects them to be

Everything that is a separate subproject should live in the subprojects
directory.

Progress on #2503

This reverts commit 451f8a8c19.

src/libutil-test-support/tests/string_callback.cc

@@ -0,0 +1,11 @@
+#include "string_callback.hh"
+
+namespace nix::testing {
+
+void observe_string_cb(const char * start, unsigned int n, void * user_data)
+{
+    auto user_data_casted = reinterpret_cast<std::string *>(user_data);
+    *user_data_casted = std::string(start);
+}
+
+}