improve reliability of TestConnPoolWaitCap
improve reliability of TestConnPoolWaitCap

It turns out the previous test succeeded only if both goroutines
tried to get a connection from the pool concurrently. Make the
test more reliable by sending three queries in parallel so that
one should succeed, one should be blocked getting a connection
from the pool, and the third should always fail.

Signed-off-by: Michael Demmer <4b836671cce77b1b2fbe63e5b2a1661879d8af44@slack-corp.com>

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