Improve test coverage

[mirekys]

• Adds @babel/plugin-transform-runtime to support async/await in tests
• Adds test:coverage npm script
• Adds dev module tests, improves overridable & store coverage

Coverage report:

----------------|---------|----------|---------|---------|------------------------------------
File            | % Stmts | % Branch | % Funcs | % Lines | Uncovered Line #s                  
----------------|---------|----------|---------|---------|------------------------------------
All files       |   76.52 |    65.22 |   72.41 |   77.68 |                                    
 dev.js         |   61.43 |    38.46 |   57.89 |   62.69 | 99-138,158-164,174-175,180-182,187 
 index.js       |       0 |        0 |       0 |       0 |                                    
 overridable.js |     100 |      100 |     100 |     100 |                                    
 store.js       |     100 |      100 |     100 |     100 |                                    
----------------|---------|----------|---------|---------|------------------------------------

Test Suites: 3 passed, 3 total
Tests:       25 passed, 25 total

Only Dev mode overridable IDTag-related code remains untested - not directly exposed, rendered via portal (may be hard to test properly with enzyme)

[mirekys]

I could also do migration to RTL, if you'd agree on this one #14 (comment), and then try update ci to setup & run tests for newer react versions