automation: execute powershell when connecting
For some reason, the ability to execute PS scripts appears to
come online after the ability to execute regular command scripts.
This is creating race conditions when connecting to instances
resulting in our wait_for_winrm() returning before PS is available
leading to an exception being thrown in other code.
Let's change the client connection code to execute a minimal
PS script so we can try to trap the exception in wait_for_winrm().
#
# Avoid interference from actual test env:
unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL