Reduce Mutex test runs (#9345)

Some tests take a long time on CI:

* `--- PASS: TestRWMutex (226.49s)`
* ` --- PASS: TestRWMutex (7.13s)`

Reduce the number of runs.

Before/after locally:

```
--- PASS: TestRWMutex (20.95s)
--- PASS: TestRWMutex (7.13s)

--- PASS: TestMutex (3.01s)
--- PASS: TestMutex (1.65s)
```

pkg/dsync/drwmutex_test.go

@@ -240,7 +240,7 @@ func HammerRWMutex(gomaxprocs, numReaders, numIterations int) {
 // Borrowed from rwmutex_test.go
 func TestRWMutex(t *testing.T) {
 	defer runtime.GOMAXPROCS(runtime.GOMAXPROCS(-1))
-	n := 1000
+	n := 100
 	if testing.Short() {
 		n = 5
 	}