Add more unit-tests for the src/core/primitives.js file

[Snuffleupagus]

Add unit-tests for various "error branches" in the code, which brings code coverage of this file to almost 100 percent.
Also, add a couple of unit-tests for the Ref.fromString functionality since it was only indirectly tested.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.20%. Comparing base (a570239) to head (8dd03be).

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #21820   +/-   ##
=======================================
  Coverage   90.20%   90.20%           
=======================================
  Files         264      264           
  Lines       67322    67321    -1     
=======================================
  Hits        60729    60729           
+ Misses       6593     6592    -1     

Flag	Coverage Δ
browsertest	66.31% <100.00%> (-0.01%)	⬇️
fonttest	8.91% <100.00%> (+<0.01%)	⬆️
integrationtest	69.40% <100.00%> (-0.01%)	⬇️
unittest	58.39% <100.00%> (+0.01%)	⬆️
unittestcli	56.94% <100.00%> (+0.01%)	⬆️

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[Snuffleupagus]

/botio browsertest

[moz-tools-bot]

From: Bot.io (Windows)

---

Received

Command cmd_browsertest from @Snuffleupagus received. Current queue size: 0

Live output at: http://54.193.163.58:8877/c03ae22db781d28/output.txt

[moz-tools-bot]

From: Bot.io (Linux m4)

---

Received

Command cmd_browsertest from @Snuffleupagus received. Current queue size: 0

Live output at: http://54.241.84.105:8877/a17a901e99d7b86/output.txt

[moz-tools-bot]

From: Bot.io (Linux m4)

---

Success

Full output at http://54.241.84.105:8877/a17a901e99d7b86/output.txt

Total script time: 18.11 mins

• Regression tests: Passed

[moz-tools-bot]

From: Bot.io (Windows)

---

Success

Full output at http://54.193.163.58:8877/c03ae22db781d28/output.txt

Total script time: 23.50 mins

• Regression tests: Passed

[calixteman]

LGTM. Thank you.

[timvandermeij]

More tests can never hurt; thank you!