Split regression and performance tests

This patch splits the regression and performance tests. Now, two files
exist under ts/tests. Regression tests enabled by default.

To enable performance tests just rename the file to .ts The testing
suite will run over all files ending with .ts

* ts/tests/0001_plan.ts. regression tests
* ts/tests/0002_plan.ts.disabled. performance tests
Rename avg_elapsed_time to elapsed_time

The new code doesn't calculate the average. Just the elapsed time.

* src/pf.lua
* src/pf_test.lua