If your app is running in English language, use this:
onView(withContentDescription("Navigate up")).perform(click());
To make it run on any language, use this:
onView(withContentDescription(R.string.abc_action_bar_up_description)).perform(click());
Note that R.string.abc_action_bar_up_description
comes from the AppCompat support library.
Also note that there is no other unique id
for the 'Arrow Back' button, because Espresso sees it as this:
+------> AppCompatImageButton {id=-1, desc=Navigate up, visibility=VISIBLE,
width=84, height=68, has-focus=false, has-focusable=false, has-window-focus=true,
is-clickable=true, is-enabled=true, is-focused=false, is-focusable=true,
is-layout-requested=false, is-selected=false,
layout-params = android.support.v7.widget.Toolbar$LayoutParams@1af06e3d,
tag=null, root-is-layout-requested=false, has-input-connection=false, x=0.0, y=0.0}