tap/awk: account for unusual korn shell signal handling behaviour

tap/awk: account for unusual korn shell signal handling behaviour

This change has been motivated by a testsuite failure on Debian
with the AT&T Korn Shell version 93u-1.

* lib/tap-driver.sh: Temporarily ignore some common signals when
waiting for the test command to complete, to avoid premature exit
in Korn shells that propagate to themselves signals that have
killed a child process.

See also related commit `v1.11-1342-g6321ad3'.