Fix flaky TestConnPrepareContext

[mjl-]

TestConnPrepareContext checks it receives an context.DeadlineExceeded. However,
the context isn't necessarily always expired. This causes most of the test runs
to fail for me, only occasionally succeeding. This change ensures the context
used in the test is actually context.DeadlineExceeded. The negative duration
causes the context package to return an already canceled context.

[mjl-]

This is the result I am seeing (on lib/pq master, with go1.17.2 linux/amd64) without this PR applied:

$ PGHOST=localhost go test -count=1 -run TestConnPrepareContext github.com/lib/pq
--- FAIL: TestConnPrepareContext (0.01s)
    --- FAIL: TestConnPrepareContext/context.WithTimeout_exceeded (0.00s)
        conn_test.go:1854: conn.PrepareContext() unexpected nil err got = <nil>, exp
FAIL
FAIL    github.com/lib/pq       0.009s
FAIL

For completeness, I'm starting my postgres like this:

export PG=/usr/lib/postgresql/13
$PG/bin/initdb -D postgres13
$PG/bin/postgres -D postgres13 -p 5432 -k '' 2>&1 | tee postgres13/postgres.log
$PG/bin/createdb -h localhost pqgotest