test(instance): update scratch storage test with instance max capacity

[Mia-Cross]

The instance API now automatically resizes the scratch volume to the instance's maximum capacity on attach. This PR updates the expected values in the scratch storage test to prevent non-empty plan after apply, and changes the instance type for the one that has the smallest capacity.

This should fix the nightly tests for the instance package.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 2.34%. Comparing base (342cbc6) to head (1ec91c5).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff            @@
##           master   #3601      +/-   ##
=========================================
- Coverage    2.34%   2.34%   -0.01%     
=========================================
  Files         445     445              
  Lines       48555   48555              
=========================================
- Hits         1140    1138       -2     
- Misses      47325   47326       +1     
- Partials       90      91       +1     

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