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:
https://github.com/mesonbuild/meson/issues/8727

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)
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