detectHostComponentNames: render test tree inside act to avoid timer leaks

[jsnajdr]

Fixes a bug that I discovered while working on #1366: when first calling render in a test suite, the render function calls detectHostComponentNames, which mounts a testing tree with a few components. This tree is rendered outside the act environment, and therefore schedules a setImmediate callback to process effects. This scheduled callback then has an unwanted interaction with my actual testing code. If there is a waitFor inside my test, the effects scheduled by my testing code will not be processed by my setImmediate, but by the one scheduled by detectHostComponentNames. That makes the precise order of events inside my test non-deterministic and unpredictable.

Look at this call stack:

and note how it starts in my waitFor.test.ts test, in the render function, then calls detectHostComponentNames, schedules setImmediate callback, and how that setImmediate later processes an update scheduled by my test code (the Apple component).

This PR avoids that by wrapping the render with act. That way, all effects scheduled by the render will come into the actQueue, and will be flushed before returning. No leaks.

In the code, I had to do some silly IIFE magic to make TypeScript happy: it doesn't know that act will synchronously and reliably initialize the result value.

[mdjastrzebski]

@jsnajdr Looks good! Again thanks for submitting this non-trivial fix!

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (3eebab2) 96.13% compared to head (fd1396c) 96.14%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1371   +/-   ##
=======================================
  Coverage   96.13%   96.14%           
=======================================
  Files          49       50    +1     
  Lines        3314     3318    +4     
  Branches      491      492    +1     
=======================================
+ Hits         3186     3190    +4     
  Misses        128      128           

Impacted Files	Coverage Δ
src/flushMicroTasks.ts	100.00% <100.00%> (ø)
src/helpers/host-component-names.tsx	100.00% <100.00%> (ø)
src/render-act.ts	100.00% <100.00%> (ø)
src/render.tsx	100.00% <100.00%> (ø)
src/waitFor.ts	76.47% <100.00%> (+0.43%)	⬆️

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[mdjastrzebski]

Released in v12.0.1 🎉