Ensure BTR waits for blocks to complete execution

[agubler]

Type: bug

The following has been addressed in the PR:

• There is a related issue
• All code has been formatted with prettier as per the readme code style guidelines
• Unit or Functional tests are included in the PR

Description:

Ensure that blocks executing during BTR are completed before capturing the rendered HTML and CSS.

[codecov]

Codecov Report

Merging #121 into master will increase coverage by 0.08%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #121      +/-   ##
==========================================
+ Coverage   83.89%   83.97%   +0.08%     
==========================================
  Files          38       38              
  Lines        1552     1560       +8     
  Branches      385      385              
==========================================
+ Hits         1302     1310       +8     
  Misses        250      250

Impacted Files	Coverage Δ
src/build-time-render/BuildTimeRender.ts	99.13% <100%> (+0.03%)	⬆️

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