Add comprehensive unit tests for empty interval handling

Add unit tests to validate all changes in interval_template.h for empty
interval handling. Tests cover:

- is_less_than_eq() and is_less_than() methods with empty intervals
- operator<=() and operator==() with empty intervals
- approx_union_with() with empty intervals (identity property)
- Edge cases: empty with empty, singleton, top, bounded intervals
- Mathematical properties: empty set as subset, equality of empty sets
- Regression tests for previously failing scenarios

Author: tautschnig

unit/Makefile

@@ -169,6 +169,7 @@ SRC += analyses/ai/ai.cpp \
        util/interval/subtract.cpp \
        util/interval/to_string.cpp \
        util/interval_constraint.cpp \
+       util/interval_template.cpp \
        util/interval_union.cpp \
        util/irep.cpp \
        util/irep_sharing.cpp \