Move jit testing utils to their own file (#21491)
Summary:
This moves `JitTestCase` to its own file so that we can have other jit
test files (ex. `test_jit_py3.py`)
There aren't any code changes, just a move and cleaning up the imports
Pull Request resolved: https://github.com/pytorch/pytorch/pull/21491
Pulled By: driazati
Differential Revision: D15703060
fbshipit-source-id: 6082e8b482100bb7b0cd9ae69738f1273e626171
