misc/cgo/testsanitizers: buffer the signal channel in TestTSAN/tsan11

misc/cgo/testsanitizers: buffer the signal channel in TestTSAN/tsan11

This fix is analogous to the one in CL 407888.

'go vet' catches the error, but it is not run on this file because the
file is (only) compiled when running testsanitizers/TestTSAN.

Fixes #53113.

Change-Id: I74f7b7390a9775ff00a06214c1019ba28846dd11
Reviewed-on: https://go-review.googlesource.com/c/go/+/409094
Auto-Submit: Bryan Mills <bcmills@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Bryan Mills <bcmills@google.com>