kunit: Don't use a managed alloc in is_literal()

There is no need to use a test-managed alloc in is_literal().
The function frees the temporary buffer before returning.

This removes the only use of the test and gfp members of
struct string_stream outside of the string_stream implementation.

Signed-off-by: Richard Fitzgerald <rf@opensource.cirrus.com>
Reviewed-by: David Gow <davidgow@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>