parallelize git-annex test for 25% speedup

Note the very weird workaround for what appears to be some kind of tasty
bug, which causes a segfault. This is not new to this modification,
I was seeing a segfault before at least intermittently when limiting
git-annex test -p to only run a single test group.

Also, the path from one test repo to a remote test repo used to be
"../../foo", which somehow broke when moving the test repos from .t to
.t/N. I don't actually quite understand how it used to work, but
"../foo" seems correct and works in the new situation.

Test output from the concurrent processes is not yet serialized.
Should be easy to do using concurrent-output.

More test groups will probably make the speedup larger. It would
probably be best to have a larger number of test groups and divvy them
amoung subprocesses numbered based on the number of CPU cores, perhaps
times 2 or 3.

Sponsored-by: Dartmouth College's Datalad project

doc/todo/speed_up___34__standalone_build__34___and__47__or_tests/comment_8_b06360071e05df18b895322382357b61._comment

@@ -0,0 +1,17 @@
+[[!comment format=mdwn
+ username="joey"
+ subject="""comment 8"""
+ date="2022-03-14T18:52:09Z"
+ content="""
+I have implemented parallelism as described in comment 7. 
+
+Currently there are 5 child processes, and the test runtime
+dropped from 444 to 334 seconds on my laptop.
+Splitting up the test groups further, so there are more child
+processes will probably improve that more.
+Remains to be seen if it helps on NFS much..
+
+The `git-annex test` output is currently a mess, it needs to be serialized.
+Ran out of time to do that today, but the speed improvement is worth
+temporarily ugly output.
+"""]]