CICD: Add stringer

[tlimoncelli]

This should fix the build problem.

CC @cafferata

[cafferata]

Looks good!

Questions:

1. Why is the testcase TXT leading ws commented out?

2. Is there another way to ensure the Go package stringer? The command go install golang.org/x/tools/cmd/stringer@latest is now on two places:

   1. dnscontrol/documentation/adding-new-rtypes.md

      Line 21 in b5010e7

      go install golang.org/x/tools/cmd/stringer@latest

   2. .github/workflows/pr_test.yml

My locally reproducible case: "stringer": executable file not found

export GOPATH=""

go generate ./...

pkg/diff2/diff2.go:3: running "stringer": exec: "stringer": executable file not found in $PATH
pkg/txtutil/txtcode.go:1: running "stringer": exec: "stringer": executable file not found in $PATH
providers/capabilities.go:1: running "stringer": exec: "stringer": executable file not found in $PATH

[cafferata]

Perhaps change the GitHub pull request title to CICD: Add stringer so that it is properly included in the release changelog.

[tlimoncelli]

Questions:

1. Why is the testcase TXT leading ws commented out?

It's more of a reminder to add that text in the future. A number of providers fail the test and would need to be fixed or have their auditrecords.go be updated.

2. Is there another way to ensure the Go package stringer? The command go install golang.org/x/tools/cmd/stringer@latest is now on two places:

   1. dnscontrol/documentation/adding-new-rtypes.md

      Line 21 in b5010e7

      go install golang.org/x/tools/cmd/stringer@latest

   2. .github/workflows/pr_test.yml

That was a typo. Its now in 1 place.

Tom