Run test tasks in foreground

[BCSharp]

Run test tasks sequenially in foreground if -jobs 0. The default did not change: it still runs all (up to the processor count) task groups in parallel jobs. -jobs 1 also runs all tasks sequentially, but because it is run as a PowerShell job, the test output is not real-time, but displayed in 10 sec intervals.

This change has two purposes:

1. To be able to monitor the test progress in real-time, without the filter. This is useful when one or a small subset of tasks is run in a query, and especially useful if some tests that operate on stdin and std out misbehave.
2. On some devcontainer systems, PowerShell's Start-Job does not work if not run by root. -jobs 0 avoids Start-Job altogether.

[slozier]

Nice!