Refactor GitHub validation workflow definition

Simplification of the GitHub validation workflow for the project by extracting the logic that creates Hummingbot conda environment and compiles and installs Hummingbot client into its own action. The validation workflow was also modified to not install miniconda. That step is not required since all GitHub virtual machines include miniconda already. Change the command used to run all tests. Instead of specifying the test module to include the command now specifies the test folders to exclude, in order to include everything else. To do this the library exclude-nose has been added to the prerequisites. Moved MockPaperExchange class out of the test packages and into the model packages for it to be included in the Hummingbot library wheel file. This will allow all tests that require the mock to be executed even in plugin repositories.

GitHub PR: https://github.com/hummingbot/hummingbot/pull/5212