tests: refactor the handling of Perl

Let's make a difference between places where Perl is required for the
test (AT_PERL_REQUIRE), and the places where it's used to run the
test, but it's not not to run the test (AT_PERL_CHECK).

* tests/local.at (AT_REQUIRE): New.
(AT_PERL_CHECK, AT_PERL_REQUIRE): New.
Use them where appropriate.

* tests/local.mk ($(TESTSUITE)): Beware not to start the line with
'-pi' if Perl is empty, as Make understands this as "it's ok to fail".
Which it is not.

tests/calc.at

@@ -619,7 +619,7 @@ AT_CHECK([cat stderr], 0, [expout])
 # Make sure we did not introduce bad spaces.  Checked here because all
 # the skeletons are (or should be) exercized here.
 m4_define([AT_CHECK_SPACES],
-[AT_CHECK(["$PERL" -ne '
+[AT_PERL_CHECK([-ne '
   chomp;
   print "$ARGV:$.: {$_}\n"
     if (# No starting/ending empty lines.
@@ -628,8 +628,8 @@ m4_define([AT_CHECK_SPACES],
         || /\s$/
         # No tabs.
         || /\t/
-        )' $1 || exit 77
-])dnl
+        )' $1
+])
 ])