Change predicable path usage under the /tmp dir

I found two places where piglit creates files with a predicable path under the /tmp directory.

This creates the opportunity for a malicious program to potentially create a file in those locations an cause issues for the piglit user.

One of the use cases in piglit was generating mako templates for the summary html command. @dbaker suggested that we might use the XDG_CACHE_HOME for these templates instead. This is done in the first two patches.

In the third patch, I changed the mako templates for the generated_tests to be created under the build directory rather than in the /tmp directory.