C++ API: Fix OOB read in unit test (#2774)

C++ API: Fix OOB read in unit test (#2774)

There were two typos in the unit tests that caused OOB accesses. Instead
of doing `d_solver.mkConst(CONST_BITVECTOR, std::string("101"), 6)`, the
closing parenthesis was in the wrong place resulting in
`std::string("101", 6)`. The second argument to `std::string(const
char*, size_t)` says how many characters to copy and results in
undefined behavior if the number is greater than the length of the
string, thus the OOB access. The commit fixes the typo and removes one
of the tests because it should not actually fail (16 is an accepted
base).