Update test for SuccessiveRHS parameter

Issue/feature addressed

The current CI does not properly test for the SuccessiveRHS parameter because the exact solution is used as initial condition for the test, such that no iteration is actually performed.

Proposed solution

Change the initial condition of the test.

Implementation

Tests

Suggested reviewers

Please suggest any people who would be appropriate to review your code.

Notes

Please add any other information that could be useful for reviewers.

Checklist

• [ ] Functions and classes, or changes to them, are documented.
• [ ] User guide/documentation is updated.
• Changelog is updated.
• Suitable tests added for new functionality.
• [ ] Contributed code is correctly formatted. (See the [contributing guidelines](https://gitlab.nektar.info/nektar/nektar/-/blob/master/CONTRIBUTING.md#using-clang-format)).
• [ ] License added to any new files.
• No extraneous files have been added (e.g. compiler output or test data files).