tests: posix: fs: declare entry statically

qemu_x86/atom was failing tests/posix/fs in
test_fs_readdir_threadsafe for some reason and only for that
platform.

All other platforms pass the testsuite properly with the dirent
declared on the stack.

Declare the dirent object statically so that qemu_x86/atom will
pass test cases.

Signed-off-by: Chris Friedt <cfriedt@tenstorrent.com>