Walk query files recursively in tree-sitter test.

We were only walking one level of depth into the `queries/` folder
during invocations of `test`, which made us attempt to open folders
rather than recurse into them.

We have to pull in the `walkdir` crate, which is required for
cross-platform walking of directories.

Fixes #938.

cli/src/error.rs

@@ -2,6 +2,7 @@ use super::test_highlight;
 use std::fmt::Write;
 use std::io;
 use tree_sitter::{QueryError, QueryErrorKind};
+use walkdir;
 
 #[derive(Debug)]
 pub struct Error(pub Vec<String>);
@@ -121,3 +122,9 @@ impl From<String> for Error {
         Error::new(error)
     }
 }
+
+impl From<walkdir::Error> for Error {
+    fn from(error: walkdir::Error) -> Self {
+        Error::new(error.to_string())
+    }
+}