Unit tests in Carthage project

[ilammy]

Add unit test targets and schemes into Carthage project of ObjCThemis. This makes it much easier to iterate on ObjCThemis since now you don't need to do a silly dance with pushing commits and pulling changes in order to just run your unit tests.

Now all you need to do is open ObjCThemis.xcodeproj, hack on ObjCThemis, then select "Test Themis (Swift 4/5, macOS/iOS)" scheme and press ⌘U. That's it. No Internet connection required (after carthage bootstrap to pull OpenSSL dependency).

Alternatively, you can run the tests from command-line:

xcodebuild \
    -derivedDataPath DerivedData  \
    -project ObjCThemis.xcodeproj \
    -scheme "Test Themis (Swift 5, macOS)" \
    test

[...]

Test Suite 'Test Themis (Swift 5, macOS).xctest' passed at 2020-03-20 16:43:02.167.
	 Executed 45 tests, with 0 failures (0 unexpected) in 4.090 (4.131) seconds
Test Suite 'All tests' passed at 2020-03-20 16:43:02.169.
	 Executed 45 tests, with 0 failures (0 unexpected) in 4.090 (4.133) seconds
2020-03-20 16:43:02.436 xcodebuild[54596:907204] [MT] IDETestOperationsObserverDebug: 4.709 elapsed -- Testing started completed.
2020-03-20 16:43:02.436 xcodebuild[54596:907204] [MT] IDETestOperationsObserverDebug: 0.000 sec, +0.000 sec -- start
2020-03-20 16:43:02.436 xcodebuild[54596:907204] [MT] IDETestOperationsObserverDebug: 4.709 sec, +4.709 sec -- end

Test session results, code coverage, and logs:
	themis/DerivedData/Logs/Test/Test-Test Themis (Swift 5, macOS)-2020.03.20_16-42-28-+0200.xcresult

** TEST SUCCEEDED **

The changes are all in Xcode XML stuff so you'd probably want to pull the branch to your machine and try it out.

Checklist

• Change is covered by automated tests ^a
• The coding guidelines are followed
• Public API has proper documentation
• Changelog is updated (maybe later, if we feel like it during the release)

^a We did not have Carthage tests before so Bitrise does not test them. I'll look into it after this PR is merged. Can't do that in the PR itself because Bitrise.

[vixentael]

I typically switch Swift version in Xcode project target, so it makes a lot of sense to create separate targets