std:test for testing infra implementations

[hiroshige-g]

How about introducing a predefined layered API (say std:test) for testing Layered API infrastructure?

std:test does something (i.e. has a side effect when evaluated), such as incrementing a global variable.

This can be used for example:

• When std:test|https://fallbackA.com/ and std:test|https://fallbackB.com/ are imported, then std:test should be evaluated only once. In other words, this is testing that std:x|y resolution happens before module map and the key in the module map is std:x, not std:x|y.

Also, this can be used to test that importing std:x actually works (i.e. evaluates std:x's script) in a simple fashion.