Split tests into "large" and "small"

[DavePearce]

Instead of splitting tests based on their name, it probably makes sense to split them based on the number of tests. So, for example, we have these files:

test.accepts
test.rejects
test.small.accepts
test.small.rejects
test.large.accepts
test.large.rejects

Here, test.accepts and test.rejects are always tested. However, tests.large.accepts and tests.large.rejects are only tested upon demand (e.g. in the CI pipeline).