CLI tests are now ported over to the new std.Build API and thus work
properly with concurrency.

 * add `std.Build.addCheckFile` for creating a
   `std.Build.CheckFileStep`.
 * add `std.Build.makeTempPath`. This function is intended to be called
   in the `configure` phase only. It returns an absolute directory path,
   which is potentially going to be a source of API breakage in the
   future, so keep that in mind when using this function.
 * add `std.Build.CheckFileStep.setName`.
 * `std.Build.CheckFileStep`: better error message when reading the
   input file fails.
 * `std.Build.RunStep`: add a `has_side_effects` flag for when you need
   to override the autodetection.
 * `std.Build.RunStep`: add the ability to obtain a FileSource for the
   directory that contains the written files.
 * `std.Build.WriteFileStep`: add a way to write bytes to an arbitrary
   path - absolute or relative to the package root. Be careful with this
   because it updates source files. This should not be used as part of
   the normal build process, but as a utility occasionally run by a
   developer with intent to modify source files and then commit those
   changes to version control. A file added this way is not available
   with `getFileSource`.