Use a more stable path to the diesel executable in tests

The test executable has moved from `target/debug/test_exe` to
`target/debug/deps/test_exe`, while the diesel executable is still in
the same directory as before. Rather than relying on this unstable
relationship, we can at least lower our surface area of change by only
relying on the path to the diesel executable. It should be fine to rely
on just `debug` here, as you would never cross-compile tests.

Author: sgrif

diesel_cli/tests/support/command.rs

@@ -69,8 +69,9 @@ impl CommandResult {
 }
 
 fn path_to_diesel_cli() -> PathBuf {
-    env::current_exe().unwrap()
-        .parent().unwrap()
+    Path::new(&env::var_os("CARGO_MANIFEST_DIR").unwrap())
+        .join("target")
+        .join("debug")
         .join("diesel")
 }