Fix the test case added by commit 95db11fdb1 (ctest: Include ChangeId in
Update.xml, 2019-04-23) to work in a release branch of CMake.
The `GIT_EXECUTABLE` value is not set before we add this test in
release branch builds.
Use the word "Update" instead of "Test" in ctest_update's RunCMake tests.
This change was prompted because I noticed that UpdateChangeId-check.cmake
was not actually getting called during the relevant test case. This file
also had to be updated since its purpose is to detect an expected XML element
(not an attribute).
