Fix test for mo_expression (!70)

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## What is the bug
Tests in for loop are all the same.
## How do you fix it


## How urgent is the bugfix
- [ ] I need it as soon as possible
- [x] I can wait for a couple of days
- [ ] None of my current codes is directly affected

## Mandatory steps before review
- [x] Gitlab CI passes _(Hint: use `make format` for linting)_ 
- [x] Bugfix is covered by additional unit tests
- [x] Mark the merge request as ready by removing `Draft:`

## Mandatory steps before merge
- [x] Test coverage does not decrease
- [ ] Reviewed by a maintainer
- [ ] Incorporate review suggestions
- [ ] Remember to edit the commit message and select the proper changelog category (feature/bugfix/other)

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Approved-by: Jonas Jucker <jonas.jucker@env.ethz.ch>
Merged-by: Jonas Jucker <jonas.jucker@env.ethz.ch>
Changelog: bugfix

test/fortran/test_expression.f90

@@ -83,7 +83,7 @@ CONTAINS
     DO i = 1, 3
       WRITE (label, '(A,I1)') 'TEST_exp_sin_z_sfc_', i
       CALL TAG_TEST(label)
-      CALL ASSERT_EQUAL(.TRUE., ALL(SIN(z(:, 1, :)*z_sfc) == val_3D(:, 1, :)))
+      CALL ASSERT_EQUAL(.TRUE., ALL(SIN(z(:, i, :)*z_sfc) == val_3D(:, i, :)))
     END DO
 
     ! clean-up