Clone repos at runtime

[Chris Warburton]

When a test fails, we want to be able to fix the problem and re-run the
test; ideally without rebuilding everything. That's why we use string
paths to shellscripts, for example, rather than paths which would cause
them to get imported into the Nix store: the latter would make running
the test again use the *old* version of the script.

We should try to do this with git repos too: rather than using fetchgit,
which will cause our test to run on the same version until a rebuild, we
should do the clone at runtime.