Use derivation hash to invalidate results

[Chris Warburton]

Since each test is a Nix derivation, they each have a hash (i.e. the
filename of the /nix/store/*.drv file).

We want to invalidate results when changes are made, e.g. to particular
files/directories or repos. Currently we try to guess when something
should be invalidated, by comparing repo names to test names; this is
both tedious and error-prone.

Instead, we can give each test a directory in the 'pass' directory. To
see if a test needs to run, we get its .drv file (e.g. with
nix-instantiate) and see if a corresponding symlink exists in the
relevant 'pass' directory. If so, this derivation has already passed, so
it doesn't need to re-run.

If no such symlink exists, we run the test (maybe with
'nix-store --realise' rather than 'nix-build', since we already know the
.drv at this point), and if successful we put a symlink in the relevant
pass directory.

We don't need to track whether particular hashes have failed, just mark
the name as failing (like we already do). If we're asked to run a test,
we get its hash and find a pass symlink for that hash, then we can
unconditionally remove the test name from the failed directory.