Improve test coverage of tax

Problem/Motivation

There are three tests for testing tax configuration components:

• Creation of tax type
• Edition of tax type
• Deletion of tax type

We're missing tests for:

• Tax rates
• Tax rate amounts
• Unexpected user input testing

Proposed resolution

Improve test coverage by creating tests for tax rates, tax rate amounts and try test unexpected user input form submissions.

Also, the tax type tests are at the moment inconsistent with existing testing structure.

Remaining tasks

1. Refactor tax type tests
2. Create tests for creating, editing and deleting tax rates
3. Create tests for creating, editing and deleting tax rate amounts
4. Improve all tests by trying unexpected user input for all editing tests
5. Review