Add support for act helper function

[jihchi]

According to the doc from testing library:

act

This is a light wrapper around the react-dom/test-utils act function. All it does is forward all arguments to the act function if your version of react supports act.

This PR adds support for the function.

[codecov-io]

Codecov Report

Merging #17 into master will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #17      +/-   ##
==========================================
+ Coverage   98.87%   98.90%   +0.02%     
==========================================
  Files           1        1              
  Lines          89       91       +2     
==========================================
+ Hits           88       90       +2     
  Misses          1        1              

Impacted Files	Coverage Δ
src/ReactTestingLibrary.re	98.90% <100.00%> (+0.02%)	⬆️

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[wyze]

Looks good, thanks again!