Document visibility settings (#1)

* Add specs for visibility setting

specs.md

@@ -34,6 +34,7 @@ Your autograder's output should be in the file results.json, in the following fo
 { "score": 44.0, // optional, but required if not on each test case below
   "execution_time": 136, // optional, seconds
   "output": "Text relevant to the entire submission", // optional
+  "visibility": "after_due_date" // Optional visibility setting
   "tests": // Optional, but required if no top-level score
     [
         {
@@ -43,6 +44,7 @@ Your autograder's output should be in the file results.json, in the following fo
             "number": "1.1", // optional (will just be numbered in order of array if no number given)
             "output": "Giant multiline string that will be placed in a <pre> tag and collapsed by default", // optional
             "tags": ["tag1", "tag2", "tag3"] // optional
+            "visibility": "visible" // Optional visibility setting
         },
         // and more test cases...
     ]
@@ -51,3 +53,31 @@ Your autograder's output should be in the file results.json, in the following fo
 
 For Java and Python, we have helper libraries that integrate with
 JUnit and unittest to produce this output format easily.
+
+### Controlling Test Case Visibility
+
+You can hide some or all test cases based on your desired conditions. Visibility
+can be controlled by setting the "visibility" field at the top level for an
+assignment, or for an individual test.
+
+Options for the visibility field are as follows:
+
+- `hidden`: test case will never be shown to students
+- `after_due_date`: test case will be shown after the assignment's due date has passed
+- `after_published`: test case will be shown only when the assignment is explicitly published from the "Review Grades" page
+- `visible` (default): test case will always be shown
+
+If an assignment level visibility setting is set, a test can override this
+setting with its own visibility setting. For example, you may set
+`"visibility":"after_due_date"` at the top level so that all tests are hidden
+until after the submission deadline. Then, you can set an individual test to
+have `"visibility":"visible"` if it should always be shown. For example, this
+can be useful for pre-submission checks such as a test that checks whether the
+student's code compiled successfully or not. Another possibility is having a
+subset of tests always visible to guide students through the homework, while
+keeping the set of tests that they will be graded on hidden until after the
+assignment is due.
+
+If test cases are hidden, students will not be able to see their total
+score. Test cases with visibility set to `hidden` don't affect this, since they
+should only be used for tests which don't contribute points to the total.