tests: add -pedantic for --enable-gcc-warnings.

This should help to avoid some portability problems.  For example,
it would have revealed the empty unions fixed by the last patch
* configure.ac (WARN_CFLAGS_TESTS, WARN_CXXFLAGS_TEST): Implement.
* tests/synclines.at (AT_TEST_SYNCLINE): Avoid -pedantic warning
about an empty translation unit.
(cherry picked from commit 5422d56a718805a2470558808355e04182a12d65)