Hook for setting GTEST_OUTPUT="xml:./"

This is currently part of B09googletests:

export GTEST_OUTPUT="xml:./"

This sets the environment so that the tests that use it will save results to junit xml file. However, B09googletests should not be run for packages which already run tests during package build (see seb128/fginther conversation below).

So, we need a hook that runs prior to package build that sets the environment variable.

[Rational to not use B09googletests]
<seb128> fginther, we need your help ;-) do you know about upstream CI/B09googletests?
* elsawang is now known as elsawang_afk
<seb128> fginther, that's breaking some of the merges
<fginther> seb128, I'm aware of it. I can check it out, what projects?
<seb128> fginther, indicator-messages ... but it's not the project fault
<seb128> fginther, you should run make check after the package build, those tests are already run during the build
<fginther> seb128, right, I've seen occasional problems with that hook
<seb128> in that case it's what happens
<seb128> - package build install stuff to debian/<binary>
<seb128> - make check is run
<fginther> seb128, if it's part of the build, I probably just need to remove the hook
<seb128> - make check in po dir finds new files in debian/<binary> that are not listed in po/POTFILES.in/skip
<seb128> it stops the build
<seb128> that's likely to happen on quite some packag
<seb128> we shouldn't run make check after make install
<seb128> that's buggy
<seb128> fginther, dh9 automatically run make check target on package build, why do we have a hook doing that as well (but at the wrong time of the build)?