Add very basic test framework for parser tests.

Start with a shell script that runs .cton files through 'cton-util cat' and
compares the output to a reference. This can get fancy later.

tests/parser/README.rst

@@ -0,0 +1,9 @@
+Parser tests
+============
+
+This directory contains test cases for the Cretonne IL parser.
+
+Each test case consists of a `foo.cton` input file and a `foo.ref` reference
+output file. Each input file is run through the `cton-util cat` command, and the
+output is compared against the reference file. If the two are identical, the
+test passes.