Verify proper owner for `pip.installed`. Refs #6912

Added test case which verifies that the owner of the `pip.installed` package is the one provided by `runas`.
`pip.installed` ownership testcase using `user` instead of `runas`. Refs #6912.
If name is empty it should not be parsed for name and version.
Updated test case to install from a requirements file.
Minor formatting fixes.
Merge pull request #6956 from s0undt3ch/issues/6912

Test case for #6912. Bugfix on requirements parsing.