Move `test/NightlyRuns` to `test/ci_tests` and add `test/ci_tests/dev` sub-directory

[lawrenceabird]

Restructure the testing directory to better reflect the use of the ISSM NightlyRuns tests in this repo. Since we don't run these tests on a nightly basis, it's more accurate to refer to these as "CI tests" here.

Adding a /dev subdirectory allows all other tests that are A) in development; B) not run in the CI, to be stored for use/testing. Additional directories can be added as needed, both within ci_tests and above.

.
.
└── test
    ├── <ADDITIONAL FOLDER>
    ├── assets
    └── ci_tests
        ├── <ADDITIONAL FOLDER>
        └── dev

We may need to revisit then when more functionality is added and/or more tests are added. For now, we can update CI.yml to isolate the ranges for the AD / non-AD tests, as follows:

  # Job 4: Run ISSM tests
  issm_tests:
    needs: build_issm
    uses: access-nri/pyISSM/.github/workflows/issm-tests.yml@main
    with:
      build-type: "issm"
      test-args: "--range 1:999"

  # Job 5: Run ISSM AD tests
  issm_ad_tests:
    needs: build_issm_ad
    uses: access-nri/pyISSM/.github/workflows/issm-tests.yml@main
    with:
      build-type: "issm-AD"
      test-args: "--range 1000:9999"

[lawrenceabird]

See PR #144