Local testing on Mac not working (fix inside)

A common error is occuring because of incorrect SDK paths.

```bash
$ swift test
warning: could not determine XCTest paths: terminated(1): /usr/bin/xcrun --sdk macosx --show-sdk-platform-path output:
xcrun: error: unable to lookup item 'PlatformPath' from command line tools installation
xcrun: error: unable to lookup item 'PlatformPath' in SDK '/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk'

error: XCTest not available
```

You can fix this error as followed:

```bash
$ xcrun --show-sdk-path --sdk macosx
```

You might get this result:

```bash
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk
```

Now change the default SDK location by invoking the following command:

```bash
$ sudo xcode-select -switch /Applications/Xcode.app/Contents/Developer
```

If that also does not work, please take a look inside the regular SDK path:

```bash
$ ls -lat /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/
```

Normally you should see the SDKs inside this directory. If not, you'll need to download the SDKs first.