Because creation of a symlink can fail on Windows with an Access Denied
error (https://learn.microsoft.com/en-us/windows/security/threat-protection/security-policy-settings/create-symbolic-links)
any tests that need a symbolic link "skip" if they run into this problem.

This change factors out a "setupSymbolicLink()" routine to make this
clearer, a bit tighter, and easier to use in future tests.

I also collapsed the "symlink in parent directory" test into the existing
"Dir.readlink" test, because the latter uses the more comprehensive
testWithAllSupportedPathTypes wrapper.