DRILL-6328: Adding unit testing docs.

closes #1220

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+# OperatorFixture
+
+Drill provides a simple way to test internal components without a full Drill server (assuming, of course, that the component is structured in a way that allows such testing; something that only a few parts of the code allow at the moment.)
+
+Testing is based on the `OperatorFixture` which sets up the basic internal mechanisms:
+
+* Memory allocator
+* Operator context
+* Fragment context (but without the server-related `DrillbitContext`)
+
+Here is a very simple example to allow testing a UDF that uses VarChar holders, which in turn use DrillBuf, which requires access to a memory allocator:
+
+```
+  @Test
+  public void testDupItFn() throws Exception {
+    try (OperatorFixture fixture = OperatorFixture.standardFixture()) {
+      OperatorContext context = fixture.operatorContext(null);
+      try {
+        // Test code goes here
+      } finally {
+        context.close();
+      }
+    }
+  }
+```
+
+In the above, we simply create an instance of the `OperatorFixture` in a try-with-resources block so we can be sure that the memory allocator shuts down, even if the test fails.
+
+Then, since this test needs an `OperatorContext`, we go ahead and create one of those. Since it isn't auto-closeable, we run tests in a try/finally block so we can be sure it is closed.
+
+Then, our code that run tests (in this case, that allocates a buffer using the managed allocator) goes in the test block.
+
+Tests that don't need an operator context are even simpler. See Drill code for examples.