Improve build_spec for tests of type 'file'

[mrcjkb]

The fix in #48 attempts to find the top-level describe node and runs that.
This might not be completely fool-proof, since it doesn't work if a single module has multiple top-level specs.

I'll have to take some extra time to think about how to get that working. Adding a --match for each test could be an option, but [...] it could exceed the maximum command line size in large test files.
Although it should be okay if I limit it to describes that don't have a parent describe.

Originally posted by @mrcjkb in #45 (comment)

[mrcjkb]

Should be done after #49.