tests: Work around a bug in Meson 0.58.

Meson 0.58 doesn't accept an array within an array for the environment
parameter to test(). See:

Work around that by using "+" to join environment lists instead.

Fixes glvnd/libglvnd#218
13 jobs for meson-test-environment-workaround in 3 minutes and 53 seconds (queued for 7 seconds)