Add dedicated pretty-printer tests with new pp-exact directive

Add a new src/test/pretty directory to hold just source files for testing the
pretty-printer.

Add a new pp-exact directive. When this directive is followed by a file name
it specifies a file containing the output that the pretty-printer should
generate. When pp-exact is not followed by a filename it says that the file
should pretty-print as written.

src/test/pretty/example2.pp

@@ -0,0 +1,3 @@
+// pp-exact:example2.pp
+
+fn main() { }