I often use git grep to search around my files because I’m more used to it than other searching tools (such as regular grep) and it’s recently become more common for me to want to exclude a pattern from my search.
$ git grep FindThisThing ":(exclude)*_test.go"
The git grep command accepts a <pathspec> as an argument, and in order to exclude a pattern. Above, exclude is the “magic word” in the pathspec to say “ignore test files”.