misc/cgo/testsanitizers: use buffered channel in tsan12.go

os/signal.Notify requires that "the caller must ensure that c has
sufficient buffer space to keep up with the expected signal rate"
as it does a nonblocking send when it receives a signal. The test
currently using a unbuffered channel, which means it may miss the
signal if the signal arrives before the channel receive operation.

Fixes #52998.

Change-Id: Icdcab9396d735506480ef880fb45a4669fa7cc8f
Reviewed-on: https://go-review.googlesource.com/c/go/+/407888
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Cherry Mui <cherryyz@google.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

misc/cgo/testsanitizers/testdata/tsan12.go

@@ -22,7 +22,7 @@ import (
 import "C"
 
 func main() {
-	ch := make(chan os.Signal)
+	ch := make(chan os.Signal, 1)
 	signal.Notify(ch, syscall.SIGUSR1)
 
 	if err := exec.Command("true").Run(); err != nil {