Add support for a "working-directory" parameter

This commit updates the tests to check that this works and also rewrites the
test script from Bash to Perl.

Co-authored-by: Dave Rolsky <autarch@urth.org>

test-project/subcrate/src/main.rs

@@ -0,0 +1,3 @@
+fn main() {
+    println!("Hello, world (subcrate)!");
+}