These tests will race access to __global_context_for_test, which can
cause the test suite to flake. Even though only a single test actually
references the variable, the logic in try_select consumes it.
This test harness ensures consistent non-blocking semantics and
validates that we have solved the toctou condition.
The __global_context_for_test is a bit of a hack to fuse together the
test supplied proc and the executing logic in packaage chan.
Jack Mordaunt
pkova
Feoramund