tests: do not assume the object file extension is .o * tests/specflg7.test: Dig out the object file extension from the OBJEXT makefile variable. * tests/substref.test: Likewise. * tests/specflg8.test: Likewise. Also check that the false-true object is created instead of checking the true-true object twice. * tests/suffix8.test: Add a chain rule for the case where the object file extension is .obj.