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/tcl/package.py

@@ -55,7 +55,7 @@ def setup_environment(self, spack_env, env):
     def build_directory(self):
         return 'unix'
 
-    @AutotoolsPackage.sanity_check('install')
+    @run_after('install')
     def symlink_tclsh(self):
         with working_dir(self.prefix.bin):
             symlink('tclsh{0}'.format(self.version.up_to(2)), 'tclsh')