Running unit test target on XCode9 returns "Early unexpected exit" error

Question

I'm learning how to add unit tests to an objective-c project using XCode9. So I've created a command line project from scratch called Foo and afterwards I've added a new target to the project called FooTests. Afterwards I've edited Foo's scheme to add FooTests. However, whenever I run the tests (i.e., menu "Product" -> "Tests" ) XCode9 throws the following error:

Showing All Messages
Test target FooTests encountered an error (Early unexpected exit, operation never finished bootstrapping - no restart will be attempted)

However, when I try to run tests by calling xcode-build from the command line, it seems that all unit tests are executed correctly. Here's the output;

a483e79a7057:foo ram$ xcodebuild test -project foo.xcodeproj -scheme foo
2020-05-15 17:39:30.496 xcodebuild[53179:948485]  IDETestOperationsObserverDebug: Writing diagnostic log for test session to:
/var/folders/_z/q35r6n050jz5fw662ckc_kqxbywcq0/T/com.apple.dt.XCTest/IDETestRunSession-E7DD2270-C6C2-43ED-84A9-6EBFB9A4E853/FooTests-8FE46058-FC4A-47A2-8E97-8D229C5678E1/Session-FooTests-2020-05-15_173930-Mq0Z8N.log
2020-05-15 17:39:30.496 xcodebuild[53179:948484] [MT] IDETestOperationsObserverDebug: (324DB265-AD89-49B6-9216-22A6F75B2EDF) Beginning test session FooTests-324DB265-AD89-49B6-9216-22A6F75B2EDF at 2020-05-15 17:39:30.497 with Xcode 9F2000 on target <DVTLocalComputer: 0x7f90b2302ef0 (My Mac | x86_64h)> (10.14.6 (18G4032))
=== BUILD TARGET foo OF PROJECT foo WITH CONFIGURATION Debug ===

Check dependencies

=== BUILD TARGET FooTests OF PROJECT foo WITH CONFIGURATION Debug ===

Check dependencies

Test Suite 'All tests' started at 2020-05-15 17:39:30.845
Test Suite 'FooTests.xctest' started at 2020-05-15 17:39:30.846
Test Suite 'FooTests' started at 2020-05-15 17:39:30.846
Test Case '-[FooTests testExample]' started.
Test Case '-[FooTests testExample]' passed (0.082 seconds).
Test Case '-[FooTests testPerformanceExample]' started.
/Users/ram/development/objective-c/foo/FooTests/FooTests.m:36: Test Case '-[FooTests testPerformanceExample]' measured [Time, seconds] average: 0.000, relative standard deviation: 84.183%, values: [0.000006, 0.000002, 0.000001, 0.000002, 0.000001, 0.000001, 0.000001, 0.000001, 0.000001, 0.000001], performanceMetricID:com.apple.XCTPerformanceMetric_WallClockTime, baselineName: "", baselineAverage: , maxPercentRegression: 10.000%, maxPercentRelativeStandardDeviation: 10.000%, maxRegression: 0.100, maxStandardDeviation: 0.100
Test Case '-[FooTests testPerformanceExample]' passed (0.660 seconds).
Test Suite 'FooTests' passed at 2020-05-15 17:39:31.589.
     Executed 2 tests, with 0 failures (0 unexpected) in 0.742 (0.743) seconds
Test Suite 'FooTests.xctest' passed at 2020-05-15 17:39:31.589.
     Executed 2 tests, with 0 failures (0 unexpected) in 0.742 (0.744) seconds
Test Suite 'All tests' passed at 2020-05-15 17:39:31.590.
     Executed 2 tests, with 0 failures (0 unexpected) in 0.742 (0.745) seconds
** TEST SUCCEEDED **

Does anyone know how to add unit tests to an xcode9 project for a command line application? If you happen to know, what's the right way of doing this and what am I doing wrong?