Add unit tests for gov.nasa.jpf.util.FixedBitSet

[HritikRaj2]

Description

The utility class gov.nasa.jpf.util.FixedBitSet lacks unit tests in the src/tests directory. This class provides efficient bit manipulation for fixed-size sets, which is critical for performance-sensitive JPF components.

Goal

Create gov.nasa.jpf.test.util.FixedBitSetTest to verify:

• Basic Operations: set(), get(), clear().
• Cardinality: cardinality() (counting set bits).
• Capacity: Ensuring the size is fixed and handles boundaries correctly.
• Bulk Operations: and(), or() if applicable.

Benefit

Ensures reliability of low-level bit set operations used throughout the system.