Improve ansible-test environment checking between tests. (#46459)

* 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.

test/runner/versions.py

@@ -0,0 +1,15 @@
+#!/usr/bin/env python
+"""Show python and pip versions."""
+
+import os
+import sys
+
+try:
+    import pip
+except ImportError:
+    pip = None
+
+print(sys.version)
+
+if pip:
+    print('pip %s from %s' % (pip.__version__, os.path.dirname(pip.__file__)))