AmplifyTests fail to execute in pipeline

[palpatim]

Describe the bug

With the addition of Xcode 12 support, the AmplifyTests bundle fails to execute in our CircleCI pipeline.

To Reproduce
Steps to reproduce the behavior:

1. Attempt to build Amplify in the pipeline (e.g., make a commit to the latest main branch)

This is also reproducible locally with a booted simulator

sim_id=$(xcrun simctl list | grep -i booted | grep -ie "[-A-F0-9]\{36\}" -o)
destination="platform=iOS Simulator,id=${sim_id}"
xcodebuild test -enableThreadSanitizer NO -workspace Amplify.xcworkspace -scheme Amplify -sdk iphonesimulator -destination "${destination}" -only-testing AmplifyTests

Expected behavior
All targets build and all unit tests pass

Actual behavior
All targets build, most unit tests pass, but the AmplifyTest bundle fails to execute, with the following error:

Testing failed:
	AmplifyTests:
		xctest (88696) encountered an error (Early unexpected exit, operation never finished bootstrapping - no restart will be attempted. (Underlying Error: Test runner exited before starting test execution. If you believe this error represents a bug, please attach the result bundle at <path to result>))

Environment(please complete the following information):

• Amplify Framework Version: main branch as of 6979763
• Dependency Manager: Cocoapods
• Swift Version : 5.3

Device Information (please complete the following information):

• Device: Simulator
• iOS Version: iOS 14
• Specific to simulators: Unknown

[palpatim]

Changing the -enableThreadSanitizer flag from NO to YES (or omitting the flag altogether) causes the test to pass. I'm bumfuzzled.

[palpatim]

Removing the enableThreadSanitizer flag from the AmplifyTests scheme allows the command line build to pass regardless of the -enableThreadSanitizer argument.

It's possible this is a bug in Xcode where the command line and scheme flags conflict. However, I am unable to reproduce the behavior in a sample project, so I'm not willing to say just now that it's a bug in Xcode as opposed to our configuration, or CocoaPods, or something else entirely.

In the meantime, I'm removing the -enableThreadSanitizer NO flag from the CircleCI config. We'd really prefer thread checking in the unit tests, so this is probably the right thing to do anyway.

[palpatim]

Resolved by #783