Convert tests to valid C++ code

Convert tests to valid C++ code

Issue #201 (bdwgc).

* tests/leak_test.c (main): Add explicit casts of void* pointer
(returned by malloc) to the types of the variables the relevant
pointers are assigned to.
* tests/realloc_test.c (main): Likewise.
* tests/smash_test.c (main): Likewise.
* tests/test.c (run_one_test): Likewise.
* tests/test.c (typed_test): Rename "new" local variable to "newP".