Rename mock exec script and make it executable

- That way its implementation can change without modifying the
  executables call site in the code.

Author: graphcareful

src/secrets/exec.rs

@@ -234,11 +234,9 @@ mod tests {
 
     fn make_test_backend(protocol: ExecVersion) -> ExecBackend {
         let command_path = PathBuf::from(env!("CARGO_MANIFEST_DIR"))
-            .join("tests/behavior/secrets/mock_secrets_exec.py");
+            .join("tests/behavior/secrets/mock_secrets_exec");
         ExecBackend {
-            command: ["python", command_path.to_str().unwrap()]
-                .map(String::from)
-                .to_vec(),
+            command: [command_path.to_str().unwrap()].map(String::from).to_vec(),
             timeout: 5,
             protocol,
         }

tests/behavior/secrets/mock_secrets_exec.py renamed to tests/behavior/secrets/mock_secrets_exec

@@ -1,3 +1,5 @@
+#!/usr/bin/env python3
+
 """
 Mock secret manager implementation used for testing the secrets.exec backend type.
 This program is meant to be exec'ed by a unit test so that the implementation can be tested.