unit-wipe-test: Skip DIO tests when the file system doesn't support O_DIRECT.

Otherwise the test fails with -EINVAL when the source tree (and thus $FILE) is on a file system such as tmpfs which doesn't support O_DIRECT. This appears to be the case on the Debian build daemons.

Not so thrilled however about the reduced code coverage in that case, especially since the test is skipped from the autopkgtest where we use CRYPTSETUP_PATH=/sbin (the tests run in an environment where the various cryptsetup* packages are installed, but in an unbuilt source tree). What is the reason for skipping the test in that case? AFAICT it uses libcryptsetup not the cryptsetup executables, and removing that guard the test appears to run fine with the system library (and CRYPTSETUP_PATH=/sbin)