Speed up ApiQuerySiteinfoTest:testContinuation

Speed up ApiQuerySiteinfoTest:testContinuation

The test fills $wgUrlProtocols with 86000 entries, forging oversized
strings which ends up eventually truncated to 100 bytes.  The aim is to
overflow $wgAPIMaxResultSize to trigger an apiwarn-truncatedresult
warning.

The test takes 230 ms on my machine and when generating code coverage
almost a minute.

A faster approach is just to lower the $wgAPIMaxResultSize limit to
trigger the warning.

Test plan:

  $ php tests/phpunit/phpunit.php \
      tests/phpunit/includes/api/ApiQuerySiteinfoTest.php \
      --filter testContinuation

Bug: T234016
Change-Id: I3dc96d394d8b98dda8f151d14544b829c92f9b10