showNotification holds back new-mail notifications for a visible,
focused tab, and the Test notification button is always pressed in one,
so the test never appeared (only its sound played). The button now
passes force, which skips that check; new mail keeps it.
Port of coffey-labs/ihasmail#51, which fixescoffey-labs/ihasmail#48.
(cherry picked from commit 28822cd2648371049e8795ef9c51a7c815b4053a)
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Same fix as coffey-labs/ihasmail#51 (for coffey-labs/ihasmail#48): the Test notification button now actually shows one.