Quickstart: use short test stanza

In addition, document convention for expect tests.

Signed-off-by: Etienne Millon <me@emillon.org>

doc/quick-start.rst

@@ -250,10 +250,7 @@ Write this in your ``dune`` file:
 
 .. code:: scheme
 
-    (alias
-     (name    runtest)
-     (deps    (:my-prog my-test-program.exe))
-     (action  (run %{my-prog})))
+    (test (name my_test_program))
 
 And run the tests with:
 
@@ -261,6 +258,18 @@ And run the tests with:
 
     dune runtest
 
+It will run the test program (the main module is ``my_test_program.ml``) and
+error if it exits with a nonzero code.
+
+In addition, if a ``my_test_program.expected`` file exists, it will be compared
+to the standard output of the test program and the differences will be
+displayed. It is possible to replace the ``.expected`` file with the last output
+using:
+
+.. code:: bash
+
+    dune promote
+
 Building a custom toplevel
 ==========================