Simplified CellTest and added a helper method for styles

[zhukoff74]

This is:

• a bugfix
• a new feature
• refactoring
• additional unit tests

Checklist:

• Changes are covered by unit tests
  • Changes are covered by existing unit tests
  • New unit tests have been added
• Code style is respected
• Commit message explains why the change is made (see https://github.com/erlang/otp/wiki/Writing-good-commit-messages)
• CHANGELOG.md contains a short summary of the change and a link to the pull request if applicable
• Documentation is updated as necessary

Why this change is needed?

• Added the createSolidFillStyle() method to eliminate duplication
• Simplified the testAppliedStyleWithRange and testAppliedStyleSingleCell tests
• Simplified the testNoChangeToActiveSheet test
• Improved code readability

[oleibman]

As long as you are cleaning up this test member anyhow, would you mind changing all the assertEquals to assertSame where possible (which I'm guessing is all of them). It's a stronger test.

[zhukoff74]

Ok

[oleibman]

Also, line 120 is the only place with expectException without expectExceptionMessage. Please add the latter, so that we confirm we are receiving the exception for the right reason.

[oleibman]

Thank you for your contribution.