Fix rare concurrency bug in test code

In the test "RPS Adapts in Response to Throttling", the assert part
of the unit test becomes unblocked after `doneStopping()` unblocks,
which happens after the second `StopQueryWithContext` calls IS ABOUT
TO return.

Usually the test was working correctly where the stopper code for
the second `StopQueryWithContext` call would fully execute. However,
very occasionally, the assertions start running before all the stopper
code runs, resulting in `stopAdapter.AssertExpectations(t)` failing
at line 2215.

This commit fixes that.

mgr_test.go

@@ -2186,11 +2186,11 @@ func TestQueryManager_Query(t *testing.T) {
 		stopAdapter.
 			On("increase").
 			Return(true).
-			Once()
+			Maybe() // Increase RPS after second StopQuery. (May happen after test ends.)
 		stopAdapter.
 			On("value").
 			Return(99_999.5).
-			Once() // Calculate minDelay after RPS increase.
+			Maybe() // Calculate minDelay after RPS increase. (May happen after test ends.)
 		stopAdapter.
 			On("value").
 			Return(99_999.5).