mail: tests: reset signal disposition before testing

As a good citizen of the OS, mail(1) won't try to catch signals that
were ignored when it started and it won't reconfigure its signal mask
unless it's going to try and handle a signal. The test should start
mail(1) off in a well-known state for the signal that it's trying to
test in order to get the behavior that's desired.

No functional change, just improves the resilience of the test.

Reviewed by: des
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D50599