try and add an xdef to mimic what go toolset does for testing.Testing…

[paulgmiller]

See https://pkg.go.dev/testing#Testing Go test sets an -X so that code knows if its in a UT

What type of PR is this? Feature?

What does this PR do? Why is it needed? Adds an xdef to mimic go test more precisely.

Should we only do for go > 1.23?

Which issues(s) does this PR fix?

Fixes #3686

Other notes for review