Prevent duplicate conversion and purchase event tracking

[mikkamp]

Changes proposed in this Pull Request:

Because of a strict comparison check for the _gla_tracked meta we were allowing the conversion and purchase snippets to show again if the order thank you page is refreshed (not loaded from cache).

This PR converts the meta value to an integer before doing the strict comparison. I've also added some unit tests to confirm that the tracking snippet isn't output in conditions where it should not. The WP proxy class had to be used to facilitate mocking of some WP functions.

Closes #2733

Detailed test instructions:

1. Setup your site fully onboarded with an Ads account to enable tracking.
2. Go through the checkout flow in an anonymous browser window until you reach the order thank you page
3. Check the page source and confirm it has two inline snippets
   1. Starting with gtag("event", "conversion"
   2. Starting with gtag("event", "purchase"
4. Hard refresh the page
5. Check the page source and confirm this time the snippets are no longer included

Changelog entry

• Fix - Prevent duplicate conversion and purchase event tracking.

[codecov]

Codecov Report

Attention: Patch coverage is 25.00000% with 6 lines in your changes missing coverage. Please review.

Project coverage is 67.0%. Comparing base (79fcca7) to head (c43a34d).
Report is 10 commits behind head on develop.

Files with missing lines	Patch %	Lines
src/Proxies/WP.php	0.0%	4 Missing ⚠️
src/Google/GlobalSiteTag.php	50.0%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             develop   #2734     +/-   ##
===========================================
+ Coverage       66.6%   67.0%   +0.4%     
- Complexity      4691    4693      +2     
===========================================
  Files            479     479             
  Lines          19608   19612      +4     
===========================================
+ Hits           13064   13136     +72     
+ Misses          6544    6476     -68     

Flag	Coverage Δ
php-unit-tests	67.0% <25.0%> (+0.4%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
src/Google/GlobalSiteTag.php	25.6% <50.0%> (+25.6%)	⬆️
src/Proxies/WP.php	23.2% <0.0%> (-1.8%)	⬇️

[eason9487]

Thanks for fixing the issue quickly!

Confirmed that gtag snippets won't be included in the order thank you page after an order is marked as tracked. LGTM.