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

docs/runtests.md

@@ -180,6 +180,10 @@ regression test suite.
 
 Lists all test case names.
 
+## `-m=[seconds]`
+
+Set timeout for curl commands in tests
+
 ## `-n`
 
 Disable the check for and use of valgrind.