Add unit testing for lexical graph to increase coverage

[oussamahansal]

Description of changes:

Add unit testing for lexical graph to increase coverage

resolves #128

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[acarbonetto]

coverage result:

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 =======================

current main:

------------------------------------------------------------------------------------------------------------------------------------------
TOTAL                                                                                                       12001   7369  38.60%
Coverage HTML written to dir htmlcov
Required test coverage of 30% reached. Total coverage: 38.60%
======================= 398 passed, 2 warnings in 35.89s =======================