* Add unified diff output to environment validation.
This makes it easier to see where the environment changed.
* Compare Python interpreters by version to pip shebangs.
This helps expose cases where pip executables use a different
Python interpreter than is expected.
* Query `pip.__version__` instead of using `pip --version`.
This is a much faster way to query the pip version. It also more
closely matches how we invoke pip within ansible-test.
* Remove redundant environment scan between tests.
This reuses the environment scan from the end of the previous test
as the basis for comparison during the next test.
(cherry picked from commit 0dc7f38787)
