Tests: avoid race-condition in leak detection

[NachoSoto]

Fixes https://app.circleci.com/pipelines/github/RevenueCat/purchases-ios/12839/workflows/2b1e24b9-428a-40ed-bcfd-a94d7591236d/jobs/89720/steps

I noticed when downloading the .xcresult that the tests had crashed with this:

Thread 0 Crashed::  Dispatch queue: com.apple.main-thread
0   libswiftCore.dylib            	       0x10bc64589 swift::runtime::AccessSet::insert(swift::runtime::Access*, void*, void*, swift::ExclusivityFlags) + 73
1   libswiftCore.dylib            	       0x10bc647e2 swift_beginAccess + 66
2   BackendIntegrationTests       	       0x13ab37c76 default argument 1 of Expectation.toEventually(_:timeout:pollInterval:description:) + 54
3   BackendIntegrationTests       	       0x13ab37b44 closure #1 in BaseBackendIntegrationTests.verifyPurchasesDoesNotLeak() + 276 (BaseBackendIntegrationTests.swift:167)

Turns out that because of the way it was written, normally Nimble tries to print the instance during a failure:

BasePurchasesTests.swift:116: error: -[UnitTests.PurchasesConfiguringTests testSettingTheDelegateAfterInitializationSendsCachedCustomerInfo] : failed - Purchases has leaked
expected to eventually be nil, got <<RCPurchases: 0x125d13bf0>>

That crash points to a race condition inside Swift's runtime. To avoid that, this changes the expectation so Nimble would only try to print "expected true, got false", with our own provided description. Therefore making sure that it doesn't try to access the instance while it's being deallocated.

[NachoSoto]

Bonus points for also not capturing self here.

[aboedo]

didn't know it was possible to capture this as a boolean, I thought this block would capture strongly instead. How does this work? like, doesn't it need the object in order to reference when re-evaluating?

[NachoSoto]

The references are weak though (defined above). The previous implementation used an @autoclosure, so expect(purchases) was converted to expect { purchases }.
Internally it was using reflection (possibly String.init(describing:)) to print a value for that object.

Now when it generates a description from the subject of the expectation, it's just a boolean.

Let me know if that makes sense.

[codecov]

Codecov Report

Merging #2806 (7bc5165) into main (1c3bed0) will decrease coverage by 0.05%.
The diff coverage is n/a.

❗ Current head 7bc5165 differs from pull request most recent head e14d7d2. Consider uploading reports for the commit e14d7d2 to get more accurate results

@@            Coverage Diff             @@
##             main    #2806      +/-   ##
==========================================
- Coverage   86.56%   86.51%   -0.05%     
==========================================
  Files         215      215              
  Lines       15484    15472      -12     
==========================================
- Hits        13403    13386      -17     
- Misses       2081     2086       +5     

see 6 files with indirect coverage changes