Print relevant message in tests not appropriate for leak detection mode

Print relevant message in tests not appropriate for leak detection mode

* cord/tests/cordtest.c (main): If GC_get_find_leak() then print a
message that the test is not designed for leak detection mode (i.e.,
plenty of leaks could be reported).
* tests/disclaim_bench.c (main): Likewise.
* tests/disclaim_test.c (main): Likewise.
* tests/disclaim_weakmap_test.c (main): Likewise.
* tests/initsecondarythread.c (main): Likewise.
* tests/middle.c (main): Likewise.
* tests/realloc_test.c (main): Likewise.
* tests/staticrootstest.c (main): Likewise.
* tests/test_cpp.cc (main): Likewise.
* tests/threadkey_test.c (main): Likewise.
* tests/trace_test.c (main): Likewise.