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-matplotlib/package.py

@@ -95,7 +95,7 @@ class PyMatplotlib(PythonPackage):
     # depends_on('ttconv')
     depends_on('py-six@1.9.0:', type=('build', 'run'))
 
-    @PythonPackage.sanity_check('install')
+    @run_after('install')
     def set_backend(self):
         spec = self.spec
         prefix = self.prefix