pw: tests: add a test for -M with a pre-existing home directory

Previous versions of pw(8) wouldn't chmod the home directory if it
already existed prior to user creation, rendering adduser(8) -M
ineffective in some cases. Add a test to cover that situation.

PR: 280099
Reviewed by: kevans

(cherry picked from commit f7cf62cf728a942e494a5b58438600998606617a)