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/runtests.pl

@@ -2421,6 +2421,10 @@ while(@ARGV) {
         $short=1;
         $automakestyle=1;
     }
+    elsif($ARGV[0] =~ /-m=(\d+)/) {
+        my ($num)=($1);
+        $maxtime=$num;
+    }
     elsif($ARGV[0] eq "-n") {
         # no valgrind
         undef $valgrind;
@@ -2563,6 +2567,7 @@ Usage: runtests.pl [options] [test selection(s)]
   -k       keep stdout and stderr files present after tests
   -L path  require an additional perl library file to replace certain functions
   -l       list all test case names/descriptions
+  -m=[seconds] set timeout for curl commands in tests
   -n       no valgrind
   --no-debuginfod disable the valgrind debuginfod functionality
   -o variable=value set internal variable to the specified value