test: skip locklinear's lockmany test for now

Since it's been reliably failing on one of the linux-arm builders
(arm5spacemonkey) for a long time.

Updates #24221.

Change-Id: I8fccc7e16631de497ccc2c285e510a110a93ad95
Reviewed-on: https://go-review.googlesource.com/104535
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

test/locklinear.go

@@ -125,6 +125,12 @@ func main() {
 		}
 	})
 
+	if runtime.GOARCH == "arm" && os.Getenv("GOARM") == "5" {
+		// lockmany reliably fails on the linux-arm-arm5spacemonkey
+		// builder. See https://golang.org/issue/24221.
+		return
+	}
+
 	checkLinear("lockmany", 1000, func(n int) {
 		locks := make([]sync.RWMutex, n*offset+1)