kasan: Add strscpy() test to trigger tag fault on arm64

When we invoke strscpy() with a maximum size of N bytes, it assumes
that:
- It can always read N bytes from the source.
- It always write N bytes (zero-padded) to the destination.

On aarch64 with Memory Tagging Extension enabled if we pass an N that is
bigger then the source buffer, it triggers an MTE fault.

Implement a KASAN KUnit test that triggers the issue with the current
implemetation of read_word_at_a_time() on aarch64 with MTE enabled.

Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>
Signed-off-by: Vincenzo Frascino <vincenzo.frascino@arm.com>