Fetch origin in benchmark test

Before running `git checkout` on the upstream
master branch, we should also fetch that branch.

Signed-off-by: Laura Loghin <lauralg@amazon.com>

integration_tests/test_benchmark.py

@@ -21,6 +21,10 @@ def test_bench():
     _run_cargo_bench("after")
 
     # Move to upstream tip.
+    subprocess.run(
+        "git fetch {} {}".format(REMOTE, BRANCH),
+        shell=True, check=True
+    )
     subprocess.run(
         "git checkout {}/{}".format(REMOTE, BRANCH),
         shell=True, check=True