Added regression test for bimolecular reaction syntax.

When parsing bimolecular reaction syntax '<->', MCell did not properly
initialize the flag indicating if the product was a complex or not for
the reverse reaction pathway. This led to reactions being incorrectly
identified as complex resulting in a crash. Thanks a lot to Sabina
Hrabetova for bringing this to our attention. This was fixed with
commit ID 0eca73ee751 in MCell's master branch.
Refactored and simplified API of some tester functions.
Continued refactoring and simplification of tester API