Request: test framework support pathlib objects

[mwichmann]

There are some basic test setup functions like test.subdir and test.write that could easily recognize Path objects and act on them, rather than requiring a string. This would allow using pathlib operations to construct file/directory paths without first having to convert them back to strings. It would help make reading a little easier, too... for example, we now see path components presented as a list, which the framework then assembles for us, like in:

test.subdir(['rep1', 'build'], ['rep1', 'build', 'var1'])