[System.Web] Fix standalone-runner to return non-zero exit code on failure (#4562)

[System.Web] Fix standalone-runner to return non-zero exit code on failure (#4562)

* [System.Web] Fix standalone-runner to return non-zero exit code on failure

We didn't notice that test runs with standalone-runner.exe failed
because it still exited with 0 in those cases.

Fixed that and the standalone tests that showed up as failed in
System.Web.Extensions (some differences likely from referencesource
code import).

* Fix replacement logic for Consts.cs.in to use Windows paths