Some of the generated sub-test names end up being excessively long,
which doesn't mix well with them being stored as test IDs that track
test runs over time. Replacing test case names with numbers harms
their readability, so start by trimming a few that end up being 500+
bytes in length.

Change-Id: Id3b22105efc08eb1bb51436aa257682d357d662c
Reviewed-on: https://go-review.googlesource.com/c/text/+/506416
TryBot-Result: Gopher Robot <[email protected]>
Run-TryBot: Dmitri Shuralyov <[email protected]>
Auto-Submit: Dmitri Shuralyov <[email protected]>
Reviewed-by: Heschi Kreinick <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>