tests: cover more realpath scenarios

tests: cover more realpath scenarios

'realpath --relative-base --relative-to' is identical to
--relative-base=--relative-to, so the test wasn't covering what
it claimed.  Expose recent fixes for handling of // on systems
where // is distinct, and for --relative-base=/.  Add test that
exposes our design decision that --relative-base that is not a
prefix of --relative-to is a no-op (if we later change behavior,
we will also have to change that part of the test).

* tests/misc/realpath: Fix typo. Add some tests.