There's a lack of Agreement (Subject-Predicate) in your sentence. Whenever plural nouns are used it's mandatory to make an agreement with its following verb (to-be in this particular case).So the answer would be - We are software developers! - We are software testers!
Besides that I'd like to add that "Incredible" is an adjective which as a general rule it does not change based on gender, number, or tense, shall we? What you saw "Incredibles" is not longer working as an adjective, in there, the word is being used as a noun, but in general terms it is not used that way.
Software tester as a whole is a noun (profession), therefore it does change, you can read up on the rules for noun-pluralization, as follows:
The plural form of most nouns is created simply by adding the letter s.
E.G. (Software developer - Software developers), (Dog - Dogs), (Girl - Girls)
Words that end in -ch, x, s or s-like sounds, require an -es for the plural:
E.G. (Fox - Foxes), (Beach - Beaches), (Witch - Witches)
There are also irregular plural nouns
E.G. (Man-Men), (Child - Children), (Mouse - Mice)
At last, there are some words which have been adapted to the language, therefore they have different rules because mantain its Latin or Greek forms.
E.G. (Phenomenon - Phenomena), (Criterion - Criteria)
This is just a taste of the bunch of rules, there are also rules for Mass and Collective nouns.
Hope it helps! Continue being proactive and inquisitive it's good to learn from everyone everyday.