`x.py test` generates several `dot` files in the source directory root

[petrochenkov]

definite_init_indirect.dot
maybe_init_indirect.dot
maybe_uninit_indirect.dot
mut_borrowed_locals_indirect.dot

If they are necessary, then they should be generated somewhere inside the build directory.
cc @ecstatic-morse

[petrochenkov]

Repening, the dot files are generated again.

borrows_hello.dot
definite_init_hello.dot
ever_init_hello.dot
flow_has_mut_interior_hello.dot
flow_needs_drop_hello.dot
maybe_init_hello.dot
maybe_uninit_hello.dot
mut_borrowed_locals_hello.dot