cmd/go: make Script/test_regexps less flaky under load

cmd/go: make Script/test_regexps less flaky under load

With the command below, I was able to reproduce failures within the
first 50 or so runs:

	go test -c -o test && stress -p 32 ./test -test.run Script/test_regexp

When printing the full failure output, we'd see:

	BenchmarkX
	    BenchmarkX: x_test.go:13: LOG: X running N=1
	BenchmarkX/Y
	    BenchmarkX/Y: x_test.go:15: LOG: Y running N=1
	    BenchmarkX/Y: x_test.go:15: LOG: Y running N=100
	    BenchmarkX/Y: x_test.go:15: LOG: Y running N=10000
	    BenchmarkX/Y: x_test.go:15: LOG: Y running N=1000000
	    BenchmarkX/Y: x_test.go:15: LOG: Y running N=100000000
	    BenchmarkX/Y: x_test.go:15: LOG: Y running N=1000000000
	BenchmarkX/Y         	1000000000	         0.000050 ns/op
	    BenchmarkX/Y: x_test.go:15: LOG: Y running N=1
	    BenchmarkX/Y: x_test.go:15: LOG: Y running N=30
	    BenchmarkX/Y: x_test.go:15: LOG: Y running N=1207
	    BenchmarkX/Y: x_test.go:15: LOG: Y running N=120700
	    BenchmarkX/Y: x_test.go:15: LOG: Y running N=12070000
	    BenchmarkX/Y: x_test.go:15: LOG: Y running N=1000000000
	BenchmarkX/Y         	1000000000	         0.000715 ns/op

In other words, the N values aren't required to be exact. It seems like
they are cut short if the machine is under stress. That's the exact
scenario we reproduce above, since I used -p=32 on my laptop with only 4
real CPU cores.

First, don't require each line to be present. Instead, use patterns
that span multiple lines, so that we can just match the first and last
N= lines.

Second, don't require the last N= lines to be exact; simply require
them to have a reasonably large number of digits.

Fixes #36664.

Change-Id: I7a9818f1a07099fa6482a26da2ac5cbea0f8ab30
Reviewed-on: https://go-review.googlesource.com/c/go/+/215578
Run-TryBot: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>