Add create_bitmap unit test

[rory-cd]

Description

Adds unit test for the create_bitmap function from images.cpp. The new test is named "can create and free a new bitmap".

This test includes sections covering the following:

• Bitmap exists and is valid
• Bitmap has correct dimensions
• Bitmap is transparent
• Bitmap can be drawn on
• Bitmap can be freed

Note: Earlier unit tests performed on load_bitmap used the terms "created"/"creating". Since this function creates a new empty bitmap rather than loading from a file, the names/comments for the earlier tests have been renamed for clarity.

Type of change

• Unit test addition

How Has This Been Tested?

Built the test project as per unit testing instructions, running skunit_tests with all tests passed.

Testing Checklist

• Tested with skunit_tests

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have requested a review from ... on the Pull Request