TestMain() runs all tests twice, once with plaintextnames=true and once
with false. Several tests mount their own filesystem and ignore the
plaintextnames variable. It makes no sense to run them twice, so
skip execution when plaintextnames is set.
Prior to this commit, gocryptfs's reverse mode did not report correct
directory entry sizes for symbolic links, where the dentry size needs to
be the same as the length of a string containing the target path.
This commit corrects this issue and adds a test case to verify the
correctness of the implementation.
This issue was discovered during the use of a strict file copying program
on a reverse-mounted gocryptfs file system.
