Omit unit tests tagged with ‘#delay’ by default

This patch changes the default Busted configuration so that it will
skip the most time-consuming tests.  But the patch also provides a
second configuration so that we can use

    $ busted --run=all

to execute every test in the test suite.

.busted

@@ -2,5 +2,12 @@ return {
     default = {
         ROOT = "tests",
         pattern = "spec.lua$",
-    }
+        ["defer-print"] = true,
+        ["exclude-tags"] = "delay",
+    },
+    all = {
+        ROOT = "tests",
+        pattern = "spec.lua$",
+        ["defer-print"] = true,
+    },        
 }