regmap: kunit: Add some test cases and a few small

Merge series from Richard Fitzgerald <rf@opensource.cirrus.com>:

This series adds some more test cases, mainly for testing:

commit eaa03486 ("regmap: maple: Fix uninitialized symbol 'ret' warnings")
commit 00bb549d ("regmap: maple: Fix cache corruption in regcache_maple_drop()")

And the pending patch ("regmap: Add regmap_read_bypassed()")

There are also a few small improvements to the KUnit implementation.