* 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.
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