Cihazın laboratuvarda test edilmesi gerekiyor.

In Turkish, when you turn a verb into a noun with -ma/-me, whatever would normally be its direct object (or subject, in a passive construction) becomes a genitive “possessor” of that noun clause. Here:
• Cihaz → Cihazın (“of the device”)
• Cihazın test edilmesi → “the testing of the device”
So Cihazın shows that the device is what’s being tested.

The verb gerekmek is impersonal and requires a noun or noun clause as its subject—it cannot take a bare infinitive. So you can’t say “Test etmek gerekiyor.” You must nominalize “to be tested” into test edilmesi (“its being tested”) and then state the necessity:
• test edilmesi gerekiyor = “its being tested is necessary.”

1. Start with the verb root et- (from test etmek, “to test”).
2. Add the passive marker -il-, giving edil- (note the t→d assimilation).
3. Attach the verbal noun suffix -me, producing edilme-.
4. Finally, add the 3rd-person singular possessive -si, yielding edilmesi.
   Combined with test, you get test edilmesi: “its being tested.”

No. Turkish has two distinct -me/-ma suffixes:
• The nominalizer (verbal noun) attaches immediately after the verb root (before any tense/negation): e.g. gel-me (“coming,” the act of coming).
• The negative attaches before tense/aspect: e.g. gel-me-di (“did not come”).
In edilmesi, -me is the nominalizer (“the act of being done”), not negation.

The suffix -da/-de marks location “in” or “at.” Since the testing takes place inside the laboratory, you say:
• laboratuvar + -da → laboratuvarda (“in the laboratory”).

This is a passive, impersonal construction: test edilmesi gerekiyor literally means “its being tested is necessary.” In such passives, the doer is omitted. If you want to name the agent, you would switch to an active form, for example:
• Cihazı laboratuvarda test etmeliyiz. (“We need to test the device in the laboratory.”)