Testing: improve Windows test logs

[martijnvans]

The idea is to parse the XML returned by powershell over stderr, and extract the human readable part and print that.

To do this, I had to separate out the part of runCommand() that logs the command and the raw stdout/stderr into a function runAndLog(), and for runRemotelyWindows(), get it to call runCommand() directly, skipping runAndLog() in order to do its own logging.

This whole thing is just a workaround for the fact that powershell ignores -OutputFormat Text when run noninteractively.

Before: https://storage.cloud.google.com/ops-agents-public-buckets-test-logs/prod/stackdriver_agents/testing/consumer/ops_agent/presubmit_github/windows/449/20220322-104940/logs/TestLoggingAgentCrashRestart_windows-2019/main_log.txt

After: https://storage.googleapis.com/ops-agents-public-buckets-test-logs/prod/stackdriver_agents/testing/consumer/ops_agent/test/windows/1043/20220808-084805/logs/TestLoggingAgentCrashRestart_windows-2019/main_log.txt

I also checked that when there is something useful on stderr, like a "missing cmdlet" error, it does show up right, and I checked that if the XML fails to parse, that the raw XML is dumped to the log, as desired.

[martijnvans]

adding -OutputFormat Text does nothing, but I included it here because:

1. it's always bothered me how this seemingly-useful option is not called out in our code anywhere
2. if microsoft ever fixes powershell's behavior, we'll start using the fix automatically.

[github-actions]

This PR was marked stale due to lack of activity. It will be closed in 14 days.