Make Perl interpreter required for system tests

This change has no practical impact, as Perl was already required for
all system tests, this check only makes it more explicit.

(cherry picked from commit 084d72d1d5)

bin/tests/system/conf.sh.in

@@ -81,7 +81,11 @@ export PYTEST=@PYTEST@
 #
 # Interpreters for system tests detected by configure
 #
-export PERL=$(command -v "@PERL@")
+export PERL=$(command -v "@PERL@" || true)
+if ! test -x "$PERL"; then
+    echo "Perl interpreter is required for system tests."
+    exit 77
+fi
 export PYTHON=$(command -v "@PYTHON@" || true)
 
 # Load common values