tests: be really robust to Perl missing

My previous tests (with ./configure PERL=false) have been fooled by
configure, that managed to find perl anyway.  This time, I ran this on
a Fedora in Docker, without Perl.

* tests/calc.at, tests/diagnostics.at, tests/headers.at,
* tests/input.at, tests/local.at, tests/named-refs.at,
* tests/output.at, tests/regression.at, tests/skeletons.at,
* tests/synclines.at, tests/torture.at: Don't require Perl.

tests/synclines.at

@@ -97,7 +97,7 @@ m4_define([_AT_SYNCLINES_COMPILE],
 #    syncline.c:4:2: error: #error "4"
 #        4 | #error "4"
 #          |  ^~~~~
-AT_CHECK([[$PERL -p -0777 - stderr <<\EOF
+AT_CHECK([["$PERL" -p -0777 - stderr <<\EOF || exit 77
   # Remove left-hand margin.
   s/^[\d ]{6}\| //gm;