Bounds: Add more tests

[llucax]

Add more tests for corner cases, for using different types, for constructors that should fail, etc.

Should test also (so it depend on):

• Bounds: Add __post_init__ to make some validations for lower/upper relationship #11
• Bounds: Make Comparable require only __lt__ #15

References:

• Add contains function to Bounds and SystemBounds frequenz-sdk-python#962 (comment)