A simple AppleScript to start up a web server in the folder where the script is located.

README

1) Add this code to a new file in Script Editor
2) Export the script as "Application" and make sure it's code signed
3) Put "Startup.app" in the root folder of the test website (e.g. where index.html is located)
4) Make sure that "Startup.app" has Full Disk Access in System Preferences > Security & Privacy > Privacy

Startup.app

tell application "Finder" to set basePath to (POSIX path of (parent of (path to me) as string))

do shell script "cd " & basePath & "; /usr/bin/ruby -run -ehttpd . -p8080 &> /dev/null & echo $!"
set pid to the result

do shell script "/usr/bin/open http://localhost:8080 &> /dev/null &"

display dialog "Click OK when done testing in " & basePath buttons {"OK"}

do shell script "kill " & pid