runtime/trace: fix TestTraceSymbolize when GOMAXPROCS=1
CL 92916 added the GOMAXPROCS test in TestTraceSymbolize.
This test only succeeds when the value of GOMAXPROCS changes.
Since the test calls runtime.GOMAXPROCS(1), it will fails
on machines where GOMAXPROCS=1.
This change fixes the test by calling runtime.GOMAXPROCS(oldGoMaxProcs+1).
Fixes #23816.
Change-Id: I1183dbbd7db6077cbd7fa0754032ff32793b2195
Reviewed-on: https://go-review.googlesource.com/93735
Run-TryBot: David du Colombier <0intro@gmail.com> Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>