Fix run-pass tests when CARGO_TARGET_DIR is not set

Author: flip1995

tests/cargo/mod.rs

@@ -1,3 +1,4 @@
+use cargo_metadata::MetadataCommand;
 use lazy_static::lazy_static;
 use std::env;
 use std::path::PathBuf;
@@ -6,7 +7,7 @@ lazy_static! {
     pub static ref CARGO_TARGET_DIR: PathBuf = {
         match env::var_os("CARGO_TARGET_DIR") {
             Some(v) => v.into(),
-            None => "target".into(),
+            None => MetadataCommand::new().exec().unwrap().target_directory,
         }
     };
     pub static ref TARGET_LIB: PathBuf = {