kunit: string-stream-test: Avoid cast warning when testing gfp_t flags

kunit: string-stream-test: Avoid cast warning when testing gfp_t flags

Passing a gfp_t to KUNIT_EXPECT_EQ() causes a cast warning:

  lib/kunit/string-stream-test.c:73:9: sparse: sparse: incorrect type in
  initializer (different base types) expected long long right_value
  got restricted gfp_t const __right

Avoid this by testing stream->gfp for the expected value and passing the
boolean result of this comparison to KUNIT_EXPECT_TRUE(), as was already
done a few lines above in string_stream_managed_init_test().

Signed-off-by: Richard Fitzgerald <rf@opensource.cirrus.com>
Fixes: d1a0d699bfc0 ("kunit: string-stream: Add tests for freeing resource-managed string_stream")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202311181918.0mpCu2Xh-lkp@intel.com/
Reviewed-by: Rae Moar <rmoar@google.com>
Reviewed-by: David Gow <davidgow@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>