Change syntax of AM_TESTS_ENVIRONMENT to what the Automake docs prefer

Change syntax of AM_TESTS_ENVIRONMENT to what the Automake docs prefer

On closer inspection of Automake docs, this is how AM_TESTS_ENVIRONMENT
is actually meant to work; the parallel test driver is even less
compatible with the old serial test driver than I'd realised :-(

Also, according to <http://www.unix.com/man-page/POSIX/1posix/export>,
"export FOO=bar" is actually required functionality for POSIX shells,
and is not a bashism. The Autoconf documentation mentions Solaris 10
as an example of somewhere this doesn't work... but at this point
I'd prefer to say "compiling dbus requires a POSIX shell".

Reviewed-by: Philip Withnall <philip.withnall@collabora.co.uk>
Bug: https://bugs.freedesktop.org/show_bug.cgi?id=89846