Fix lots of windows tests

Fix lots of windows tests

* Add a convenience method bin() for generating the name of a binary. On windows
  this remembers to append `.exe`.

* Stop executing relative paths to binaries and relying on PATH. This is
  suffering from rust-lang/rust#15149 and failing to spawn processes on windows.
  Additionally, this allows the tests to work with a pre-installed cargo becuase
  the freshly built executables are precisely specified.

* A new function, escape_path(), was added for tests. When generated source
  files with paths, this function needs to be called to properly escape the
  \-character that appears in windows path names. Without this function we would
  be generating invalid TOML and rust.