Add generic isctest.run.EnvCmd helper to pytest

Add generic isctest.run.EnvCmd helper to pytest

A generic helper that calls the environment-specified binaries in a
developer-friendly manner, i.e. passing arguments as strings rather than
having to split them first.

The isctest.run.cmd() remains as the basis which provides a clean and
robust interface, while the isctest.run.EnvCmd() can be used as a
convenient wrapper for tests, or when there are some shared default
parameters.

The isctest.run.Dig() is superseded with the isctest.run.EnvCmd(). In
the future, we might revisit adding Dig() or command-specific helpers
again, but it probably only makes sense if they offer command-aware
attributes / methods, rather than just being shortcuts to
isctest.run.EnvCmd().