Add unit testing for lexical graph to increase coverage part 2

[oussamahansal]

Description of changes:

Add unit testing for lexical graph to increase coverage

resolves #128

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[oussamahansal]

Coverage result:

TOTAL 12001 6823 43.15% Coverage HTML written to dir htmlcov Required test coverage of 30% reached. Total coverage: 43.15% ======================= 969 passed, 1 warning in 49.08s ========================

Current main:
TOTAL 12001 7169 40.26% Coverage HTML written to dir htmlcov Required test coverage of 30% reached. Total coverage: 40.26% ======================= 655 passed, 2 warnings in 44.07s =======================