- 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
