present: add end-to-end rendering test

See testdata/README for test format.
This will make it easier to fix problems and not regress.
Markdown tests in followup CL (with bug fixes).

Change-Id: I5dedca26d3c29fd428066ffb38c6605343784a19
Reviewed-on: https://go-review.googlesource.com/c/tools/+/224957
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Andrew Bonventre <andybons@golang.org>
diff --git a/present/testdata/basic.p b/present/testdata/basic.p
new file mode 100644
index 0000000..1cda98d
--- /dev/null
+++ b/present/testdata/basic.p
@@ -0,0 +1,24 @@
+Title
+Subtitle
+
+Name
+
+* Heading
+
+Text
+on two lines.
+
+More text.
+
+---
+<h1>Title</h1>
+<h2>Subtitle</h2>
+<author>
+<p>Name</p>
+</author>
+<section>
+<h2 id="TOC_1.">Heading</h2>
+<p>Text
+on two lines.</p>
+<p>More text.</p>
+</section>