test: generate gcgort

In e0adc35c470d9bf7f462c9d26340b494cbe7ce43 test/gcgort.go is added. This test may benefit from being generated instead of hand-written to increase maintainability by reducing repeating and narrow test coverage by removing extraneous setup code like the modifier struct.

This change would be after 1.11 and my opinion is it should only be done if the test strategy is also updated for the stated garbage collector, types, and concurrency interactions coverage. Currently the test failure relies on the runtime implementation causing a panic.

From https://go-review.googlesource.com/c/go/+/93715 discussion with @aclements.