Fix various issues with testsuite code on Windows
Summary: Previously we would make direct calls to `diff` using `os.system`. On Windows `os.system` is implemented using the standard idiom `CreateProcess .. WaitForSingleObject ..`. This again runs afoul with the `_exec` behaviour on Windows. So we ran into some trouble where sometimes `diff` would return before it's done. On tests which run multiple ways, such as `8086` what happens is that we think the diff is done and continue. The next way tries to set things up again by removing any previous directory. This would then fail with and error saying the directory can't be removed. Which is true, because the previous diff code/child is still running. We shouldn't make any external calls to anything using `os.system`. Instead just use `runCmd` which uses `timeout`. This also ensures that if we hit the cygwin bug where diff or any other utility hangs, we kill it and continue and not hang the entire test and leave hanging processes. Further more we also: I...
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