cmd/compile/internal/syntax: needs regression test framework

[griesemer]

Reminder issue: The syntax package doesn't have a good setup for adding regression tests.

[griesemer]

Marking for 1.10 simply so it stays on the radar.

[griesemer]

Test case for #20789 (https://go-review.googlesource.com/#/c/46713/) could move there.

[griesemer]

Test case for #23434 (https://go-review.googlesource.com/c/go/+/87898, corresponding go/parser fix) should be added here.

[griesemer]

Test case for #23385 should be added here.

[gopherbot]

Change https://golang.org/cl/88335 mentions this issue: cmd/compile/internal/syntax: implement regression test harness for syntax errors

[gopherbot]

Change https://golang.org/cl/88336 mentions this issue: cmd/compile/internal/syntax: added test cases for recent fixes