Running python tests fails on some current conda installs

[saskeli]

This relates to building the python command to run tests with the tmc-cli.

The current conda release versions do not define the CONDA_PYTHON_EXE environment variable and some do not have the py alias defined. In case both of these fail, tmc-cli will report 0/0 test passed and no useful error information. For all reported cases python -m tmc would work but won't be run since the OS is detected to be windows.

The current development version of conda does have the environment variable set, so the issue will be partially resolved as a new version is realeased. However adding some sort of error handling to try running with python -m tmc if all else fails could be nice.

Also more transparent error reporting could be neat.