connpool: fix racy test

[vmg]

Description

--- FAIL: TestOpen (0.05s)
    pool_test.go:174: 
        	Error Trace:	/home/runner/work/vitess/vitess/go/pools/smartconnpool/pool_test.go:174
        	Error:      	Not equal: 
        	            	expected: int(5)
        	            	actual  : int64(4)
        	Test:       	TestOpen
    pool_test.go:175: 
        	Error Trace:	/home/runner/work/vitess/vitess/go/pools/smartconnpool/pool_test.go:175
        	Error:      	Not equal: 
        	            	expected: 5
        	            	actual  : 4
        	Test:       	TestOpen
FAIL
FAIL	vitess.io/vitess/go/pools/smartconnpool	15.407s

Spotted by @harshit-gangal. A sleep is not always enough to synchronize the wait. We can make the synchronization explicit by waiting until a client is in the wait queue, then offering a connection to them.

Related Issue(s)

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[mattlord]

I feel like we should use a context here to break out of this if needed... but this is a unit test so the test itself should time out relatively quickly.

[vmg]

In theory this code cannot deadlock, because the goroutine is constantly trying to add itself to the waitgroup, but you're right that the test timeout will catch this if down the road we break the implementation.