Adding test cases for Equivalent RE

All test passed 
Test on line 85 is failing. The regular expression are not equal, probably a typo. Please  fix it before we can merge.

Also run all the tests before doing the pull request. Thank you.

This is the error message.

```
======================================================================
FAIL: test_valid_tests (tests.test_regex_to_nfa.TestRegexToNfa)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/aortegag/projects/proyecto-final-fundamentos/tests/test_regex_to_nfa.py", line 85, in test_valid_tests
    "(ab*a+b)(aaa(ab*a+b)+abb*a+b+aab)*"))
AssertionError: Tuples differ: (True, 'dfa1 and dfa2 accept t... != (False, 'aa')

First differing element 0:
True
False

- (True, 'dfa1 and dfa2 accept the same language')
+ (False, 'aa')
```