Add Spine Toolbox unit and execution tests to GitHub workflows

[soininen]

This adds two new GitHub actions that are run on push: Toolbox unit tests and execution tests. These are meant for checking that changes in spinedb_api do not accidentally break Toolbox. Since they are meant to be quick "smoke tests", they are executed only on Linux host using Python 3.8.

Currently, the target branch in Toolbox is 0.8-dev. This should be changed to master once the development branches are merged.

Resolves #358

Checklist before merging

• [x] Documentation (also in Toolbox repo) is up-to-date
• [x] Release notes in Toolbox repo have been updated
• [x] Unit tests have been added/updated accordingly
• [x] Code has been formatted by black
• [ ] Unit tests pass

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (dad322e) 77.60% compared to head (2e31fdb) 77.60%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## 0.8-dev #360 +/- ## ======================================== Coverage 77.60% 77.60% ======================================== Files 75 75 Lines 9370 9370 Branches 1983 1983 ======================================== Hits 7272 7272 Misses 1770 1770 Partials 328 328 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.