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

examples/custom-controller/CMakeLists.txt

@@ -15,6 +15,8 @@ set_target_properties(${exe} PROPERTIES ENABLE_EXPORTS ON)
 set(test "${testsuite}.${name}")
 add_test(NAME ${test} COMMAND ${CMAKE_CURRENT_BINARY_DIR}/${exe})
 set_tests_properties(${test} PROPERTIES TIMEOUT 30)
-if(lsan_options)
-  set_tests_properties(${test} PROPERTIES ENVIRONMENT_MODIFICATION ${lsan_options})
+# (the previous `if(lsan_options)` here could never fire -- that variable
+# only ever existed in the add_example() function scope)
+if(env_mods)
+  set_tests_properties(${test} PROPERTIES ENVIRONMENT_MODIFICATION "${env_mods}")
 endif()