Learn French: Le réceptionniste allait tester l'ordinateur, mais il y a une panne. - The receptionist was going to test the computer, but there is a breakdown.

Using aller in the imperfect tense followed by an infinitive like tester creates a structure meaning "was going to do something". This is the past version of the near future tense, and it perfectly translates to "was going to test" in English.

Why does the sentence switch from the past tense (allait) to the present tense (il y a)?

This highlights a contrast in time. The receptionist's plan to test the computer was in the past (allait tester), but the computer's breakdown is an ongoing, current fact in the present (il y a une panne).

How would I use this lesson's grammar to say "he hasn't tested the computer yet"?

You would use the past tense (passé composé) and place pas encore between the helper verb and the past participle. It becomes: Il n'a pas encore testé l'ordinateur.

Is réceptionniste always a masculine noun?

Job titles ending in -iste are usually the exact same for both men and women. You can tell this specific receptionist is male because of the masculine article Le. If it were a female receptionist, it would simply be La réceptionniste.

What exactly does une panne mean here?