sweet: reduce length of TestSweetEndToEnd

This test is failing on the LUCI builders because it's running out the
default timeout, and timeouts for subrepos haven't been ported there

Surprisingly, this is the only repository that runs out the default
clock. This prompted me to look into where time is spent in this test,
and I noticed that the etcd benchmark didn't use the -short flag at all

In addition to making use of -short in the etcd benchmark, this change
also fiddles around with other benchmarks' use of the -short flag to
make the test slightly shorter. The net decrease in time spent on my
local machine is 25% (5m -> 3m45s).

The test is also tweaked to allow more of the small benchmarks to run in
parallel during the test.

Finally, this change adds timing logs to the test, so when it next times
out we have a better idea as to why.

Change-Id: I7e394e675fd767d13778d934584c12ce23675c01
Reviewed-on: https://go-review.googlesource.com/c/benchmarks/+/506556
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Michael Pratt <mpratt@google.com>
Run-TryBot: Michael Knyszek <mknyszek@google.com>
5 files changed