cmd/compile/internal/syntax: added test cases for recent fixes

cmd/compile/internal/syntax: added test cases for recent fixes

R=go1.11.

Now that we have a syntax error test harness, we can add the
proper tests for the recent parser fixes.

For #20800.
For #20789.
For #23385.
For #23434.

A test for #20789 already exists in test/fixedbugs, but this
is the better location for that test. But leaving the existing
one where it is as well.

Change-Id: I5937b9b63bafd1efab467a00344302e717976171
Reviewed-on: https://go-review.googlesource.com/88336
Reviewed-by: Matthew Dempsky <mdempsky@google.com>