jpm test now starts a new interpreter per test.

This should help with setup/teardown semantics, especially
with native modules.

auxbin/jpm

@@ -783,7 +783,9 @@ int main(int argc, const char **argv) {
              (case (os/stat ndir :mode)
                :file (when (string/has-suffix? ".janet" ndir)
                        (print "running " ndir " ...")
-                       (dofile ndir :exit true))
+                       (def result (os/execute [(dyn :executable "janet") ndir] :p))
+                       (when (not= 0 result)
+                         (os/exit result)))
                :directory (dodir ndir))))
          (dodir "test")
          (print "All tests passed.")))