test: add more tests for const decls with ommitted RHS expressions
Add analogous tests to go/types and types2 test suites.
Make sure "assert" built-in is available in type-checker
tests.
For #49157.
For #53585.
Change-Id: I092901ecb43eb4833c09bd8f5e38efbe0285babe
Reviewed-on: https://go-review.googlesource.com/c/go/+/414795
Run-TryBot: Robert Griesemer <gri@google.com> Reviewed-by: Ian Lance Taylor <iant@google.com> Reviewed-by: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
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