testsuite: Define _POSIX_SOURCE for tests [PR113278]
As the tests assume that fileno() is visible (only part of POSIX),
define the guard to ensure that it's visible. Currently, glibc appears
to always have this defined in C++, newlib does not.
Without this patch, fails like this can be seen:
Testing analyzer/fileno-1.c, -std=c++98
.../fileno-1.c: In function 'int test_pass_through(FILE*)':
.../fileno-1.c:5:10: error: 'fileno' was not declared in this scope
FAIL: c-c++-common/analyzer/fileno-1.c -std=c++98 (test for excess errors)