Directions on running tests is incomplete: "run_tests" does not exist in tree

[ngie-eign]

As seen in the documentation referenced below, the directions suggest that users should be able to run run_tests from the source directory. In reality, no such file exists in the repository today.

tpm/README.md

Lines 87 to 97 in 99469c4

	### Tests
	Tests for this project run on [Travis CI](https://travis-ci.org/tmux-plugins/tpm).
	When run locally, [vagrant](https://www.vagrantup.com/) is required.
	Run tests with:
	```bash
	# within project directory
	./run_tests
	```

Looking at the logic in .travis.yml, another plugin named tmux-test is required first (it's generally pulled as a git submodule). Once that's installed in the documented location, more tools become available for running the needed tests.

Having complete directions would allow me to easily confirm that TPM is installed correctly with FreeBSD.