docs(guide/Unit Testing): tighter English

Leaner, more forceful style.

Fixes a grammatical problems with subject number doesn't agrees with the verb. (originally "We have built many features into
Angular which makes...", which reduces to "features...which makes").

More authoritative use of commas, such as the Oxford comma in lists of three or more.

Sentences that are sentences, not fragments. (Yes, that was a fragment just now. I'm not writing docs now that reflect the polish of the project.)

The English grammatical/stylistic notion of parallelism.

Etc. This is just polish. The ideas are fine.

Closes #10478