fix: Execute correct %appscript and add test for instance start --app

[dtrudg]

Description of the Pull Request (PR):

Add an e2e test for instance start --app. Use the same echo server pattern that's implemented for testing instance start, but make %appstart foo a string reversing echo server, so we can tell that we started the right thing.

This uncovered issue #2410 - so we also have a fix to run the correct %appscript instead of the generic %startscript for instance start --app.

This fixes or addresses the following GitHub issues:

• Fixes e2e: scif: add test for instance start --app functionality #1944
• Fixes instance start --app does not execute correct startscript #2410

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