Remove the x-test language from the .desktop files

In cmake, this is not just a matter of running `grep -v`, unfortunately.

The regexp contains the trailing LF because otherwise I cannot ask it to remove
that line instead of insisting on replacing it with an empty one.

CCBUG: 332578
Do not install the x-test translations

The .qm files should still be produced when doing the build (especially the
debug one), but there is no need to install them to the users' systems.

BUG: 332578
Add support for the x-test language

Since we're using Qt's facilities for determining what language to use (based on
the technology we're using, we're a Qt application, not a KDE one), we end up
with examining the QLocale::system(). However, its return value is actually
sanitized by the C library (?), i.e. trying to use either LANG or LC_ALL set to
x-test or x_test does not work at all.

Rather than inventing a custom scheme, it's easier to just follow the KDE's
conventions for env vars, and this patch makes it follow the KDE_LANG env
variable when asked for the x-test language. We still follow the systemwide
settings, and the Qt's own locale is not changed (so you might still get
"untranslated" messages in Trojita in case they're provided by Qt itself).

REVIEW: 117429
SVN_SILENT made messages (.desktop file)
Merge branch 'kill-the-xtest'

REVIEW: 117423