Add a new temp file creation API to SynchronousTestCase

[exarkun]

SynchronousTestCase.mktemp is a handy feature of trial. However, it returns a temporary path as a string, and strings are a terrible abstraction for paths. A new API should be introduced which returns a FilePath instead.

When unicode path support is added to FilePath, this will also have the advantage of allowing the logic for supporting that to be kept in FilePath instead of spread throughout any tests that want a temporary file.

It might also be nice to do away with the extraneous temp segment always included in these paths, if it is possible to do so.