chore(web): Improve web/test.sh script

[ermshiperete]

• previously there was an extra --browsers flag - $BROWSERS already contains the flag
• previously we specified $KARMA_FLAGS twice if we set $KARMA_EXT_FLAGS
• some cleanup

@keymanapp-test-bot skip

[keymanapp-test-bot[bot]]

User Test Results

Test specification and instructions

User tests are not required

Test Artifacts

• Android
  • Keyman for Android apk
  • FirstVoices for Android 16.0 apk
  • FirstVoices Keyboards for Android apk
  • KeyboardHarness apk
  • Keyman for Android 16.0 apk
  • KMSample1 apk
  • KMSample2 apk
• Developer
  • Keyman Developer
  • Compiler Regression Tests
  • kmcomp.zip
• iOS
  • Keyman for iOS (simulator image)
  • FirstVoices Keyboards for iOS (simulator image)
  • TestFlight internal PR build version - 18.0.27 (0.11355.10895)
• Keyboards
  • Test Keyboards
• Web
  • KeymanWeb Test Home
• Windows
  • Keyman for Windows
  • FirstVoices Keyboards for Windows
  • Text Editor (32 bit)
  • Text Editor (64 bit)

[keyman-server]

Changes in this pull request will be available for download in Keyman version 18.0.29-alpha