Small CI cleanup

[kfcampbell]

There's a couple of small fixes to our CI checks I'd like to make:

• make build was running go install, not go build
• make test ran a fmtchecker but not our actual unit tests, which are trivial to run
• Our CONTRIBUTING.md was recommending running unit tests, which we don't really encourage for local development validation
• tools.go was improperly formatted; not sure why our CI check didn't catch it
• Removed the -mod=vendor flag
  • In this doc, see the section that reads: "At go 1.14 or higher, automatic vendoring may be enabled. If the file vendor/modules.txt is present and consistent with go.mod, there is no need to explicitly use the -mod=vendor flag."
• Updated to the most recent version of Go for the Actions runner
  • The project remains low in version and able to be importable by a wide range of Go versions.
  • This should give us quicker build times and smaller binary sizes while retaining backards compatibility.

I also added testdata/ to the .gitignore file based on a personal preference, but I'm happy to revert that if desired.

[jcudit]

Wish I could approve it twice 🙇🏾