tmf: Fix TestRefreshTextTrace failure because of bad time zone reset

tmf: Fix TestRefreshTextTrace failure because of bad time zone reset

Commit 6941941dae56 (https://git.eclipse.org/r/#/c/66579/)
changed tests to set explicitly a time zone but it doesn't
reset it properly. Because of this, TestRefreshTextTrace had
mismatching time stamps.

This can reproduced by running FilterViewerTest first then
running TestRefreshTextTrace. The easiest way is to modify
SWTBotStressTests to have this:

s.addTest(new JUnit4TestAdapter(FilterViewerTest.class));
s.addTest(new JUnit4TestAdapter(TestRefreshTextTrace.class));

To fix it, we reset the time zone to the time zone id, not the label.
The safest way to do this is to use what we decided to use in the
default preference map.

Change-Id: I9b25e2c3a458cee62b74cfde61043270b62b1c01
Signed-off-by: Marc-Andre Laperle <marc-andre.laperle@ericsson.com>
Reviewed-on: https://git.eclipse.org/r/66723
Reviewed-by: Hudson CI
Reviewed-by: Matthew Khouzam <matthew.khouzam@ericsson.com>
Tested-by: Matthew Khouzam <matthew.khouzam@ericsson.com>
Reviewed-by: Bernd Hufmann <bernd.hufmann@ericsson.com>
Tested-by: Bernd Hufmann <bernd.hufmann@ericsson.com>