Fixes exception from xmlchange

[jasonb5]

The xmlchange tool should not raise an exception when a diff is detected.

Test suite:
Test baseline:
Test namelist changes:
Test status: [bit for bit, roundoff, climate changing]

Fixes [CIME Github issue #]
User interface changes?:
Update gh-pages html (Y/N)?:

[jedwards4b]

@jasonb5 that solved the first problem but I am still seeing:

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FAIL: test_e_xmlquery (test_sys_create_newcase.TestCreateNewcase)
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Traceback (most recent call last):
File "/glade/work/jedwards/sandboxes/cesm2_x_alpha/cime/CIME/tests/test_sys_create_newcase.py", line 321, in test_e_xmlquery
self.assertTrue(output == "TRUE", msg="%s != %s" % (output, BUILD_COMPLETE))
AssertionError: False is not true : FALSE != False

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I think that I have a fix for this and will push it back if that's okay?

[jasonb5]

@jedwards4b Go ahead and push your fix.

[jedwards4b]

@jasonb5 based on this result I am ready to merge this PR.

[jedwards4b]

I'll have to double check - I realized that #4631 fixed the github testing and that I needed that more urgently than testing on derecho or perlmutter. I think there are still some issues that need to be addressed there, but I'm not sure if this is one of them.