chore: fix race with events tests

[toddbaert]

I believe this was causing intermittent failures with tests. This static field on a package private member meant that the instance we created for testing (which we shutdown) was shared with the "real" API instance (see here for an example failure).

I don't think this constitutes a production bug, but it should make our tests more reliable.

Edit: I ran this suite locally 10+ times and in GH 5 times and it seems to run reliably now.

[sonarqubecloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

No Coverage information
0.0% Duplication

[codecov]

Codecov Report

Merging #545 (9d22159) into main (c545ed5) will increase coverage by 0.13%.
Report is 1 commits behind head on main.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##               main     #545      +/-   ##
============================================
+ Coverage     95.23%   95.36%   +0.13%     
- Complexity      329      330       +1     
============================================
  Files            31       31              
  Lines           755      755              
  Branches         37       37              
============================================
+ Hits            719      720       +1     
+ Misses           19       18       -1     
  Partials         17       17              

Flag	Coverage Δ
unittests	95.36% <100.00%> (+0.13%)	⬆️

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Files Changed	Coverage Δ
...rc/main/java/dev/openfeature/sdk/EventSupport.java	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes

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[Kavindu-Dodan]

casued by the test ShutdownBehaviorSpecTest where we call executor shutdown