tests: avoid spurious failures when awk is "traditional awk" * tests/Makefile.am (do_subst): Also substitute '@AWK@'. * tests/defs-static.in ($AWK): New, user-overridable and defaulting to the substituted '@AWK@'. * tests/defs (fetch_tap_driver): When the shell+awk implementation of the TAP driver is required, export AM_TAP_AWK to point to a properly capable awk, since the default awk might be a "traditional" implementation, which won't be enough for our driver. This is the right fix, since a proper setting of AM_TAP_AWK in the driver's environment is expected (and documented to be required) in real world uses of 'tap-driver.sh'.