Fix GMT_App_M_1?.sh so it passes test

[remkos]

Description of proposed changes

Really weird. When $n == $i, this fails only in the test, it runs fine otherwise.

j1=$(expr $n - $i)

This PR bypasses this problem, introduced by #8654

Reminders

• Make sure that your code follows our style. Use the other functions/files as a basis.
• Add tests for new features or tests that would have caught the bug that you're fixing.
• Describe changes to function behavior and arguments in a comment below the function declaration.
• If adding new functionality, add a detailed description to the documentation and/or an example.