runtests: add -m=seconds to override test curl command timeout

To override the curl default of 5 minutes (300000 ms).

Sometimes a simple test data change can result in a stuck test, this
option makes it exit with an error early. Possible future use in CI
or fast machines to prevent a single test taking 5 minutes and failing
the whole job.

Example hangers:

tests/data/test65:
```diff
-<data1000 crlf="yes">
+<data1000 crlf="headers">
```

tests/data/tests993:
```diff
-%repeat[1000 x 95 328485%0d%0a]%</data>
+%repeat[1000 x 95 328485%0d%0a]%
+</data>
```

Closes #19319

tests/runner.pm

@@ -972,6 +972,9 @@ sub singletest_run {
         if((!$cmdhash{'option'}) || ($cmdhash{'option'} !~ /no-q/)) {
             $CMDLINE .= " -q";
         }
+        if($maxtime) {
+            $CMDLINE .= " --max-time $maxtime";
+        }
     }
 
     if(use_valgrind() && !$disablevalgrind) {