Make GNUPGHOME for tests overridable

* lang/python/tests/Makefile.am,
lang/qt/tests/Makefile.am,
tests/Makefile.am,
tests/gpg/Makefile.am,
tests/gpgsm/Makefile.am,
tests/opassuan/Makefile.am (GNUPGHOME): Make variable explict.

If the build directory has too long path, gpgme could fail.
This is similar to

https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=847206

In order to fix that, this patch extracts the GNUPGHOME variable
to be presented directly in the Makefile and thus overridable by
command line option.

A build system can then create a symlink to the GNUPGHOME directory
in /tmp and use that symlink as the GNUPGHOME directory
thus making the path very short.

Patch provided by vlmarek

• GnuPG-Bug-Id: T4091