tests: make runtests check that disabled tests exists

... and error out if so. Removed '536' from DISABLED as there is no such
test file.

Closes #5288
diff --git a/tests/data/DISABLED b/tests/data/DISABLED
index 9b3af1a..198728f 100644
--- a/tests/data/DISABLED
+++ b/tests/data/DISABLED
@@ -22,7 +22,6 @@
 1307
 # Pipelining is deprecated
 530
-536
 584
 1900
 1901
diff --git a/tests/runtests.pl b/tests/runtests.pl
index 1c6a028..d43b277 100755
--- a/tests/runtests.pl
+++ b/tests/runtests.pl
@@ -5492,7 +5492,13 @@
                 next;
             }
             if($_ =~ /(\d+)/) {
-                $disabled{$1}=$1; # disable this test number
+                my ($n) = $1;
+                $disabled{$n}=$n; # disable this test number
+                if(! -f "data/test$n") {
+                    print STDERR "WARNING! Non-exiting test $n in DISABLED!\n";
+                    # fail hard to make user notice
+                    exit 1;
+                }
             }
         }
         close(D);