kunit: make KUNIT_EXPECT_STREQ() quote values, don't print literals

Before:
>  Expected str == "world", but
>      str == hello
>      "world" == world

After:
>  Expected str == "world", but
>      str == "hello"
<we don't need to tell the user that "world" == "world">

Note: like the literal ellision for integers, this doesn't handle the
case of
  KUNIT_EXPECT_STREQ(test, "hello", "world")
since we don't expect it to realistically happen in checked in tests.
(If you really wanted a test to fail, KUNIT_FAIL("msg") exists)

In that case, you'd get:
>  Expected "hello" == "world", but
<output for next failure>

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