test: support an alternative runtime library dir per test

- introduce FAIRMQ_TEST_LD_LIBRARY_PATH, which prepends a directory to
  each test's environment via ctest, so the tests can run against an
  alternative runtime library (e.g. a tsan-instrumented libstdc++)
- LD_LIBRARY_PATH rather than an injected rpath: an rpath added via the
  linker flags cannot precede the rpath spack's gcc adds through its
  specs file, so the compiler's own libstdc++ would keep winning the
  runtime search order
- scoped per test on purpose: an instrumented library has unresolved
  __tsan_* symbols and must not be loaded into uninstrumented tools
  like cmake, ctest or ninja
- fail the configuration instead of silently dropping the injection on
  CMake < 3.22 (ENVIRONMENT_MODIFICATION)
- cover the example tests too; they share the instrumented runtime but
  not the locale-cache warmup (their main() is the installed public
  header). The custom-controller env block was dead before: it tested
  lsan_options, which only ever existed in the add_example() function
  scope, so the test also never received the LSan suppressions

test/CMakeLists.txt

@@ -24,9 +24,19 @@ if(definitions)
   set(definitions DEFINITIONS ${definitions})
 endif()
 
+set(test_environment)
 if(ENABLE_SANITIZER_LEAK)
   get_filename_component(lsan_supps "${CMAKE_CURRENT_SOURCE_DIR}/leak_sanitizer_suppressions.txt" ABSOLUTE)
-  set(environment ENVIRONMENT "LSAN_OPTIONS=set:suppressions=${lsan_supps}")
+  list(APPEND test_environment "LSAN_OPTIONS=set:suppressions=${lsan_supps}")
+endif()
+# Run the tests against an alternative runtime library directory, e.g. a
+# tsan-instrumented libstdc++. Set per test (not at the job level), because
+# such a library must only be loaded into instrumented executables.
+if(FAIRMQ_TEST_LD_LIBRARY_PATH)
+  list(APPEND test_environment "LD_LIBRARY_PATH=path_list_prepend:${FAIRMQ_TEST_LD_LIBRARY_PATH}")
+endif()
+if(test_environment)
+  set(environment ENVIRONMENT ${test_environment})
 endif()
 
 add_testhelper(runTestDevice