test: add basic tests for MessageChannel

[addaleax]

Checklist

• [x] make -j4 test (UNIX), or vcbuild test (Windows) passes
• [x] tests and/or benchmarks are included
• [x] commit message follows commit guidelines

Affected core subsystem(s)

test/worker

[addaleax]

@ayojs/core ping? :)

[addaleax]

Landed in 8e049c1391ca829a33c3f34d0704c930b2a226e9