net/http/pprof: make TestDeltaProfile less flaky by retrying

In some slow environment, the goroutine for mutexHog2 may not run
within 1secs. So, try with increasing seconds parameters,
and declare failure if it still fails with the longest duration
parameter (32sec).

Also, relax the test condition - previously we expected the
profile's duration is within 0.5~2sec. But obviously, in some
slow environment, that's not even guaranteed. Just check we get
non-zero duration in the result.

Update #38544

Change-Id: Ia9b0d51429a2093e6c9eb92cf463ff6952ef3e10
Reviewed-on: https://go-review.googlesource.com/c/go/+/229498


Run-TryBot: Hyang-Ah Hana Kim <hyangah@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>