Add section on tests

spec/src/_preamble.tex

···
     morekeywords=[3]{rule,is,ft,given,and,or},
     morekeywords=[4]{when,cancel,resume,invert,yield},
     morekeywords=[5]{module,import,from,use,export,at},
+    morekeywords=[6]{test,assert},
     morestring=[b]{"},
     morestring=[s]{$"}{"},
     morecomment=[l]{\#},

spec/src/overview/additional-features/documentation.tri

···
 proc main!() {
     loop {
         let inputNumber = Int readline!()
-            when 'EOF cancel break # done
+            when 'EOF cancel break  # done
             when 'NAN cancel #- default when invalid: -# 0
         println!(double inputNumber)
     }

spec/src/overview/additional-features/index.tex

···
 author desires, but more importantly, can exported locally alongside the
 documentation of all other referenced code, so it can be found in a
 predictable location alongside any project that uses it.
+
+\subsubsection{Testing}
+
+Tests are defined similarly to procedures, but using the keyword \kw{test}
+instead of \kw{proc}. A test is run as if it was the entrypoint to the
+application. If execution reaches the end successfully, the test is
+considered a success; otherwise it is a failure.
+
+\inputcode{tests.tri}
+
+To make tests easier to write, the \kw{assert} keyword will abort the
+current process when its parameter does not evaluate to \val{true}.

spec/src/overview/additional-features/tests.tri

···
+func double x = x * 2
+func triple x = x * 4  # uh oh
+
+test can_double!() {
+    assert double 2 == 4
+}
+
+test can_triple() {
+    assert triple 2 == 6
+}