Check target environment instead of Spyder environment for dependencies

[jitseniesen]

As noted in #175 (comment), the plugin uses the environment that Spyder runs in to find its dependencies (nose, pytest and their plugins). Instead, it should use the environment that the user configured for running the tests.

This happens in the following places (at least): when testing whether a test run configuration is valid, when using the plugin option menu item "Show dependencies", when checking for pytest-cov in PR #175.

[jitseniesen]

See spyder-ide/spyder-kernels#392 (comment) for a possible way to implement this.

[jitseniesen]

A problem with using the IPythonConsole plugin to check the environment is that (as far as I can tell) there is not necessarily a console open with the current environment.