move test files

examples/mut.rs

examples/mut2.rs

examples/ref.rs

examples/ref2.rs

examples/ref3.rs

examples/ref4.rs

examples/ref5.rs

examples/ref_err.rs

examples/ref_err2.rs

examples/ref_err3.rs

examples/ref_mut.rs

examples/type_err.rs

examples/type_err2.rs

examples/vm.rs

examples/while.rs

src/check.rs

src/env.rs

test_files/deref_assign2.rs

tests/test_check.rs

tests/test_vm.rs