xcrun: error: unable to find utility "xctest", not a developer tool or in PATH

Assuming you already have Xcode installed:

Start Xcode, select "Preferences -> Locations". Chances are that your Command Line Tools are not set. Select the suggested Xcode-tools location and you are done.

Additional:

You could check with:

xcode-select -p

which tools are set, mine showed:

/Library/Developer/CommandLineTools

After the location in the preferences was set, it showed:

/Applications/Xcode.app/Contents/Developer

(as it should)

Just a wrong path, all the stuff still can be done w/o invoking GUI:

~/% sudo xcode-select -s /Library/Developer/CommandLineTools
~/% sudo xcode-select -p                                    
/Library/Developer/CommandLineTools
~/% sudo xcode-select -s /Applications/Xcode.app/Contents/Developer
~/% sudo xcode-select -p                                           
/Applications/Xcode.app/Contents/Developer

The latest -s command fixes xcrun: error: unable to find utility "xctest" …

Xcode -> Preferences (or command+,)

Locations tab -> Command Line tools section: Select a tool there and you are done.