testutil: rename Calls() -> CallArgsList() for clarity

The current MockCmd.Calls() is a bit ambiguous so follow the example
of pytest here and rename to the cleaner CallArgsList() that
captures more closely what the helper is actually doing.

internal/testutil/testutil_test.go

@@ -22,7 +22,7 @@ func TestMockCommand(t *testing.T) {
 	assert.Equal(t, [][]string{
 		{"run1-arg1", "run1-arg2"},
 		{"run2-arg1", "run2-arg2"},
-	}, fakeCmd.Calls())
+	}, fakeCmd.CallArgsList())
 }
 
 func TestCaptureStdout(t *testing.T) {