tests: split out ~1/3 of test-graft.t test-graft.t is ~2400 lines and takes 34s to run. This patch moves the last ~1/3 of it to a separate file. The parts now run in 22s + 13s. On top of that, we can remove the #testcases from the old file, so it's only 22s + 2*13s instead of the 2*34s it was before. Differential Revision: https://phab.mercurial-scm.org/D7802