Test seeds are now machine-independent.

Test seeds are generated by hashing the test function name
and filename. Previously, test seeds were using the full
filename, meaning that test results were very machine dependent.
We now use the relative filename (i.e. the name within the
Nengo directory), making things machine independent.

Fixes #586.

Other changes:
- Moved `test_seed` out of `function_seed` to make
  its use more explicit.
- Added a test for the seed that should pass on all machines.