add tests

[rhoskal]

--------------------|---------|----------|---------|---------|-------------------
File                | % Stmts | % Branch | % Funcs | % Lines | Uncovered Line #s 
--------------------|---------|----------|---------|---------|-------------------
All files           |   91.67 |       84 |   85.71 |   91.67 |                   
 src                |     100 |    90.91 |     100 |     100 |                   
  selectors.ts      |     100 |    90.91 |     100 |     100 | 18                
 src/commands       |   89.04 |    78.57 |   77.78 |   89.04 |                   
  formatDocument.ts |   89.04 |    78.57 |   77.78 |   89.04 | 66-73             
--------------------|---------|----------|---------|---------|-------------------
Test Suites: 2 passed, 2 total
Tests:       1 skipped, 9 passed, 10 total
Snapshots:   0 total
Time:        0.422 s, estimated 1 s
Ran all test suites.