Make it possible to use a central directory for all tests that create a temporary file

When running the tests as part of a shared library on Android, ie an app/apk, all temporary files created have to use an absolute path and write to certain proscribed directories.  By having all these modules' tests use `std.file.deleteme` and then hardcoding an absolute path there and in `std.stdio.testFilename`, all these temporary files were created properly on Android/x86, plus anyone can set a single path in one central location in the source in the future.

I wonder if it'd be worthwhile to replace `std.stdio.testFilename` with `std.file.deleteme` also, thoughts?