execinfo tests: Strengthen tests. * tests/test-execinfo.c (test_backtrace): Add an argument. Check the return value of backtrace(). Check that backtrace_symbols_fd is defined. Check the return value of backtrace_symbols(). (main): Test also the case of a short buffer. * modules/execinfo-tests (Makefile.am): Verify that LIB_EXECINFO is defined.