kunit: fix display of failed expectations for strings

Currently the following expectation
  KUNIT_EXPECT_STREQ(test, "hi", "bye");
will produce:
  Expected "hi" == "bye", but
      "hi" == 1625079497
      "bye" == 1625079500

After this patch:
  Expected "hi" == "bye", but
      "hi" == hi
      "bye" == bye

KUNIT_INIT_BINARY_STR_ASSERT_STRUCT() was written but just mistakenly
not actually used by KUNIT_EXPECT_STREQ() and friends.

Signed-off-by: Daniel Latypov <dlatypov@google.com>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Tested-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>