Bug 1300163 - Lazy load certain mach context attributes using the 'key' mechanism, r=armenzg

You can set attributes on a mach context by using the 'key' argument to the context_handler. Basically,
whatever gets returned by the handler when <key> is passed in, will get set (i.e cached) on the context
object for fast retrieval next time. This is a way to lazy load these attributes.

Previously I was setting functions like 'find_firefox()' on the context object, and then having the
mach_commands call that directly. But this way is much cleaner.

Now, the loaded 'mozharness_config' can be stored as an attribute on the context. Also 'find_firefox()'
is now an attribute called 'firefox_bin'.

MozReview-Commit-ID: 4lsKGpizfH7

testing/xpcshell/mach_test_package_commands.py

@@ -21,12 +21,10 @@ from mach.decorators import (
 
 def run_xpcshell(context, **kwargs):
     args = Namespace(**kwargs)
+    args.appPath = args.appPath or os.path.dirname(context.firefox_bin)
     args.utility_path = context.bin_dir
     args.testingModulesDir = context.modules_dir
 
-    if not args.appPath:
-        args.appPath = os.path.dirname(context.find_firefox())
-
     if not args.xpcshell:
         args.xpcshell = os.path.join(args.appPath, 'xpcshell')