test: pre-fill libstdc++ ctype caches before threads exist

- std::ctype<char> caches narrow()/widen() results per character in
  plain char arrays of the global classic-locale facet, written without
  synchronization from header-inlined code (locale_facets.h); two
  threads exercising an uncached character concurrently (e.g. compiling
  a std::regex in Channel::Validate) constitute a true data race that
  ThreadSanitizer rightfully reports
- the stores are real and unsynchronized, so a tsan-instrumented
  libstdc++ cannot help here; instead fill the caches before any thread
  is spawned, which turns every later access into a pure read
- warm the lazily-installed num_put/num_get caches used by stream
  insertion/extraction as well, via a small format/parse round-trip
- wire the warm-up into the gtest runner main() and, via a static
  initializer, into the test device runner

test/CMakeLists.txt

@@ -32,6 +32,7 @@ endif()
 add_testhelper(runTestDevice
     SOURCES
     helper/runTestDevice.cxx
+    helper/LocaleWarmup.h
     helper/devices/TestPairLeft.h
     helper/devices/TestPairRight.h
     helper/devices/TestPollIn.h