Increase test coverage of haeley-webgl

As long as coveralls is not set-up for the repository, here is a current coverage report created by running `npm run test:nyc` on commit c8a002d:

```shell
-----------------------|---------|----------|---------|---------|-------------------
File | % Stmts | % Branch | % Funcs | % Lines | Uncovered Line #s
-----------------------|---------|----------|---------|---------|-------------------
All files | 20.2 | 10.81 | 19.3 | 19.6 |
about.ts | 100 | 100 | 100 | 100 |
allocationregister.ts | 100 | 100 | 100 | 100 |
buffer.ts | 0 | 0 | 0 | 0 | 4-188
changelookup.ts | 100 | 100 | 100 | 100 |
context.ts | 6.32 | 2.85 | 2.08 | 6.32 | 88-1194
contextmasquerade.ts | 86 | 75 | 88.88 | 86 | 116-124
extensions.ts | 100 | 100 | 100 | 100 |
extensionshash.ts | 100 | 100 | 100 | 100 |
formatbytesizes.ts | 7.89 | 0 | 0 | 7.89 | 20-146,159-181
framebuffer.ts | 0 | 0 | 0 | 0 | 4-506
geometry.ts | 0 | 0 | 0 | 0 | 6-129
gl2facade.ts | 2.47 | 0 | 0 | 2.52 | 27-464
initializable.ts | 82.25 | 100 | 82.75 | 82.69 | 127-142
object.ts | 83.33 | 100 | 66.66 | 83.33 | 105,138-147
program.ts | 0 | 0 | 0 | 0 | 4-240
renderbuffer.ts | 0 | 0 | 0 | 0 | 4-203
shader.ts | 0 | 0 | 0 | 0 | 4-198
texture2d.ts | 0 | 0 | 0 | 0 | 4-451
vertexarray.ts | 0 | 0 | 0 | 0 | 4-136
wizard.ts | 0 | 0 | 0 | 0 | 4-107
-----------------------|---------|----------|---------|---------|-------------------
```

Colored: