Fix miscellaneous bugs in fs.wrap

[althonos]

Type of changes

• Bug fix
• Tests

Checklist

• I've run the latest black with default args on new code.
• I've updated CHANGELOG.md and CONTRIBUTORS.md where appropriate.
• I've added tests for new code.
• I accept that @PyFilesystem/maintainers may be pedantic in the code review.

Description

This PR fixes the following issues:

• fs.wrap.WrapReadOnly did not wrap the removetree method, meaning a filesystem could still be wiped if this command was called. Closes WrapReadOnly does not wrap removetree #468 .
• fs.wrap.WrapCachedDir methods isdir and isfile would raise ResourceNotFound instead of returning False on non-existing paths.

I also added more tests for these two classes, and in particular a test making sure fs.copy, fs.move and fs.mirror respect WrapReadOnly semantics; this is to avoid any new undetected removal that could occur if #429 or similar improvements are added which could use os functions to improve performance in various places when given filesystems with a syspath.

[coveralls]

Coverage increased (+0.008%) to 95.329% when pulling 235b56a on readonly-removetree into 94f0323 on master.