kunit: reduce layering in string assertion macros

The current macro chain looks like:
KUNIT_EXPECT_STREQ => KUNIT_EXPECT_STREQ_MSG => KUNIT_BINARY_STR_EQ_MSG_ASSERTION => KUNIT_BINARY_STR_ASSERTION.
KUNIT_ASSERT_STREQ => KUNIT_ASSERT_STREQ_MSG => KUNIT_BINARY_STR_EQ_MSG_ASSERTION => KUNIT_BINARY_STR_ASSERTION.
<ditto for STR_NE>

After this change:
KUNIT_EXPECT_STREQ => KUNIT_EXPECT_STREQ_MSG => KUNIT_BINARY_STR_ASSERTION.
KUNIT_ASSERT_STREQ => KUNIT_ASSERT_STREQ_MSG => KUNIT_BINARY_STR_ASSERTION.
<ditto for STR_NE>

All the intermediate macro did was pass in "==" or "!=", so it seems
better to just drop them at the cost of a bit more copy-paste.

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