[parserTests] Ensure manual selser changes have a clean DOM
The test item for manual selser changes needs to be cloned from
the original item when --selser=noauto is activated from the
command line so that its DOM is not clobbered by a previous test.
Also, avoid directly changing the changes list in
applyManualChanges as that can affect following tests. To test,
pass both --wt2wt and --selser on the command line.