This patch converts gen_interpolation_tests from concatenating a ton of
strings to generating it's output with a mako template. There are
cosmetic changes to the output file.

Using mako is possibly a little faster, but mainly has the benefit of
being much more readable than a long series of string concatenations,
even at the cost of a little more code.

Signed-off-by: Dylan Baker <dylanx.c.baker@intel.com>