test: Make N-API weak-ref GC tests asynchronous

One of the N-API weak-reference test cases already had to be made
asynchronous to handle different behavior in a newer V8 version:
https://github.com/nodejs/node/pull/12864
When porting N-API to Node-ChakraCore, we found more of the test
cases needed similar treatment:
https://github.com/nodejs/node-chakracore/issues/246 So to make
thes tests more robust (and avoid having differences in the test code
for Node-ChakraCore), I am refactoring the tests in this file to
insert a `setImmedate()` callback before every call to `gc()` and
assertions about the effects of the GC.

Backport-PR-URL: https://github.com/nodejs/node/pull/19447
PR-URL: https://github.com/nodejs/node/pull/13121


Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>