tests: drop useless requirements of 'g++'

tests: drop useless requirements of 'g++'

* tests/lflags2.test ($required): Remove 'g++', as any working
C++ compiler should be enough, and ./configure will cause the
test to skip if no working C++ compiler is found.
* tests/yflags2.test: Likewise.
* tests/subobj9.test: Likewise.
* tests/silentcxx.test: Likewise.  Also, do not force depmodes
that could cause non-GNU C++ compilers to fail.
* tests/silentcxx-gcc.test: New test, like `silentcxx.test',
but forcing "fast" gcc depmode (and thus requiring the GNU C++
compiler).
* tests/specflg10.test: Add proper "fixme" comment telling that
we should make this test work with a generic C++ compiler.
* tests/Makefile.am (TESTS): Update.