Add tests for StartupReportFullyDrawn

[NotWoods]

No description provided.

[codecov-commenter]

Codecov Report

Merging #12560 into master will increase coverage by 0.05%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master   #12560      +/-   ##
============================================
+ Coverage     24.91%   24.97%   +0.05%     
- Complexity      830      836       +6     
============================================
  Files           389      389              
  Lines         15657    15657              
  Branches       2022     2022              
============================================
+ Hits           3901     3910       +9     
+ Misses        11445    11434      -11     
- Partials        311      313       +2     

Impacted Files	Coverage Δ	Complexity Δ
.../org/mozilla/fenix/perf/StartupReportFullyDrawn.kt	77.77% <100.00%> (+77.77%)	6.00 <0.00> (+6.00)
...pp/src/main/java/org/mozilla/fenix/ext/Activity.kt	66.66% <0.00%> (+33.33%)	0.00% <0.00%> (ø%)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update d4bb432...ac86644. Read the comment docs.