build systems: simpler, clearer decorators: run_after, run_before (#2860)

* PackageMeta: `run_before` is an alias of `precondition`, `run_after` an alias of `sanity_check`

* PackageMeta: removed `precondition` and `sanity_check`

* PackageMeta: decorators are now free-standing

* package: modified/added docstrings. Fixed the semantics of `on_package_attributes`.

* package: added unit test assertion as side effects of install

* build_systems: factored build-time test running into base class

* r: updated decorators in package.py

* docs: updated decorator names

var/spack/repos/builtin/packages/py-yt/package.py

@@ -65,7 +65,7 @@ class PyYt(PythonPackage):
     depends_on("py-sympy", type=('build', 'run'))
     depends_on("python @2.7:2.999,3.4:")
 
-    @PythonPackage.sanity_check('install')
+    @run_after('install')
     def check_install(self):
         # The Python interpreter path can be too long for this
         # yt = Executable(join_path(prefix.bin, "yt"))