Add regression test for #224 (#255)

* Use dune 2.7

This lets us use dune's built in cram testing, instead of the bespoke
cram-style testing that used for the spec. I think it makes sense to
support the current finicky testing setup for the spec, because it lets
us use the commonmark spec as a raw source of truth, but for further
tests we add I think we'll benefit from a more lightweight set up.

Since we are already using dune, it seems sensible to use this testing
framework we get access to with a two line change.

We might consider reworking the spec testing setup to generate
dune-supported cram tests in the future tool, as it would eliminate a
good chunk of code.

* Add regression test for #224

dune-project

@@ -1,8 +1,9 @@
-(lang dune 2.0)
-
+(lang dune 2.7)
 (name omd)
 (version 2.0.0~alpha1)
 
+(cram enable)
+
 (generate_opam_files)
 
 (license ISC)

tests/blackbox/dune

@@ -0,0 +1,2 @@
+(cram
+ (deps %{bin:omd}))

tests/blackbox/regression-224.t

@@ -0,0 +1,11 @@
+Regression test for https://github.com/ocaml/omd/issues/224
+
+  $ omd << "MD"
+  > hello_world:
+  > ```
+  > is_this_code_
+  > ```
+  > MD
+  <p>hello_world:</p>
+  <pre><code>is_this_code_
+  </code></pre>