Add more tests for bad configuration file

tests/compile-fail/conf_bad_toml.rs

@@ -0,0 +1,6 @@
+// error-pattern: error reading Clippy's configuration file
+
+#![feature(plugin)]
+#![plugin(clippy(conf_file="./tests/compile-fail/conf_bad_toml.toml"))]
+
+fn main() {}

tests/compile-fail/conf_bad_toml.toml

@@ -0,0 +1,2 @@
+fn this_is_obviously(not: a, toml: file) {
+}

tests/compile-fail/conf_path_non_string.rs

@@ -0,0 +1,6 @@
+#![feature(attr_literals)]
+#![feature(plugin)]
+#![plugin(clippy(conf_file=42))]
+//~^ ERROR `conf_file` value must be a string
+
+fn main() {}