delete test_artifacts directory when tests complete

* add std.os.deleteTree
 * add std.os.deleteDir
 * add std.os.page_size
 * add std.os API for iterating over directories
 * refactor duplication in build.zig
 * update documentation on how to run tests
8654bc1810Andrew Kelley committed on 4/20/2017, 6:26:36 AMparent1ff73a8
9 files changedLine totals unavailable