Add tests to cover minor BDD operations and operator overloads

[ssoelvsten]

Whlie looking through the diff for #610 , I found the following missing coverage. These should be tested to be working as expected, similar to the other functions.

Tasks

• bdd_invimp
• bdd_equiv
• bdd_diff
• bdd_less
• bdd::bdd() (default construction to false)
• bdd::operator |=
• bdd::operator &=
• bdd::operator ^=

Other

• The definition of the binary bdd::operators should be moved from adiar/bdd/bdd.cpp into adiar/bdd/apply.cpp.
• The definition of the binary bdd::operator== and its derivates should be moved to adiar/bdd/pred.cpp