kunit: Cover 'assert.c' with tests

kunit: Cover 'assert.c' with tests

There are multiple assertion formatting functions in the `assert.c`
file, which are not covered with tests yet. Implement the KUnit test
for these functions.

The test consists of 11 test cases for the following functions:

1) 'is_literal'
2) 'is_str_literal'
3) 'kunit_assert_prologue', test case for multiple assert types
4) 'kunit_assert_print_msg'
5) 'kunit_unary_assert_format'
6) 'kunit_ptr_not_err_assert_format'
7) 'kunit_binary_assert_format'
8) 'kunit_binary_ptr_assert_format'
9) 'kunit_binary_str_assert_format'
10) 'kunit_assert_hexdump'
11) 'kunit_mem_assert_format'

The test aims at maximizing the branch coverage for the assertion
formatting functions.

As you can see, it covers some of the static helper functions as
well, so mark the static functions in `assert.c` as 'VISIBLE_IF_KUNIT'
and conditionally export them with EXPORT_SYMBOL_IF_KUNIT. Add the
corresponding definitions to `assert.h`.

Build the assert test when CONFIG_KUNIT_TEST is enabled, similar to
how it is done for the string stream test.

Signed-off-by: Ivan Orlov <ivan.orlov0322@gmail.com>
Reviewed-by: Rae Moar <rmoar@google.com>
Acked-by: David Gow <davidgow@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>