Fix Makefile to not mix dependencies between BIN_DIR and DISTRIB_DIR/lib

In #5228, there are two FIXMEs in the makefile where a test depends on bin/libHalide.dylib and also distrib/lib/libautoschedule_*.dylib, which injects a dependency on distrib/lib/libHalide.dylib, loading two copies of libHalide.

It's currently hacked around, this needs to be fixed. Probably by making all the tests in the Makefile only depend on things in distrib