[vpython] Few more flag parsing test cases.
[vpython] Few more flag parsing test cases.

Add a few more flag parsing test cases around flag separators and Python
targets.

TBR=iannucci@chromium.org

Bug: None
Test: unit
Change-Id: Ia3864b7aef4e907a3b592516e051e39f0eb668a3
Reviewed-on: https://chromium-review.googlesource.com/868250
Reviewed-by: Daniel Jacques <8400006a9338a0893dca0101ad49838955bd6b2c@chromium.org>
Commit-Queue: Daniel Jacques <8400006a9338a0893dca0101ad49838955bd6b2c@chromium.org>

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go