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`.
0b8736f5ed