Combine CI Coverage Files Across Environments

[sco1]

This needs validation, but I believe that the coverage report currently being provided by the CI is only for whatever test environment runs the tests last. While not an immediate issue for the current codebase, as Python 3.8-specific logic starts to be added to the codebase the coverage reported by a single test environment will decrease.

To remedy this, the coverage report should be generated from the combination of reports across all of the test environments.

This will likely be easiest to work on after Microsoft adds 3.8 environment support (See: #43)