test: deflake chan/select3.go

On a slow or distracted machine, 0.1s is sometimes
not long enough for a non-blocking function call to complete.
This causes rare test flakes.
They can be easily reproduced by reducing the wait time to (say) 100ns.

For non-blocking functions, increase the window from 100ms to 10s.
Using different windows for block and non-blocking functions,
allows us to reduce the time for blocking functions.
The risk here is false negatives, but that risk is low;
this test is run repeatedly on many fast machines,
for which 10ms is ample time.
This reduces the time required to run the test by a factor of 10,
from ~1s to ~100ms.

Fixes #20299

Change-Id: Ice9a641a66c6c101d738a2ebe1bcb144ae3c9916
Reviewed-on: https://go-review.googlesource.com/47812
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

test/chan/select3.go

@@ -40,8 +40,15 @@ func testBlock(signal string, f func()) {
 		c <- never // f didn't block
 	}()
 	go func() {
-		time.Sleep(1e8) // 0.1s seems plenty long
-		c <- always     // f blocked always
+		if signal == never {
+			// Wait a long time to make sure that we don't miss our window by accident on a slow machine.
+			time.Sleep(10 * time.Second)
+		} else {
+			// Wait as short a time as we can without false negatives.
+			// 10ms should be long enough to catch most failures.
+			time.Sleep(10 * time.Millisecond)
+		}
+		c <- always // f blocked always
 	}()
 	if <-c != signal {
 		panic(signal + " block")