testsuite: Increase ulimit for T18623

1 MByte was just too tight and failed intermittently on some platforms
(e.g. CentOS 7). Bumping the limit to 8 MByte should provide sufficient
headroom.

Fixes #23139.

testsuite/tests/rts/T18623/all.T

@@ -8,7 +8,7 @@ test('T18623',
      # Recent versions of osx report an error when running `ulimit -v`
      when(opsys('darwin'), skip),
      when(arch('powerpc64le'), skip),
-     cmd_prefix('ulimit -v ' + str(1024 ** 2) + ' && '),
+     cmd_prefix('ulimit -v ' + str(8 * 1024 ** 2) + ' && '),
      ignore_stdout],
     run_command,
     ['{compiler} --version'])