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

CMakeLists.txt

@@ -46,6 +46,13 @@ include(FairMQDependencies)
 
 
 # Targets ######################################################################
+if(FAIRMQ_TEST_LD_LIBRARY_PATH AND CMAKE_VERSION VERSION_LESS 3.22)
+  # The per-test injection relies on ctest's ENVIRONMENT_MODIFICATION, which
+  # older CMake silently drops -- the tests would run against the default
+  # runtime while looking green.
+  message(FATAL_ERROR "FAIRMQ_TEST_LD_LIBRARY_PATH requires CMake >= 3.22")
+endif()
+
 if(BUILD_FAIRMQ)
   add_subdirectory(fairmq)
 endif()