This fixes of the file contents checks in checkout to give
slightly better error messages by directly calling the underlying
clar assertions so the file and line number of the top level call
can be reported correctly, and renames the helpers to not start
with "test_" since that is kind of reserved by clar.

This also enables some of the CRLF tests on all platforms that
were previously Windows only (by pushing a check of the native
line endings into the test body).