Fix a stack trace caused by environment variables in test setups.
Fix a stack trace caused by environment variables in test setups.

An example trace:

[snip]
>   File "/usr/lib/python3.6/concurrent/futures/thread.py", line 55, in run
>     result = self.fn(*self.args, **self.kwargs)
>   File "/home/trhd/Projects/meson/mesonbuild/mtest.py", line 221, in run_single_test
>     child_env.update(self.options.global_env.get_env(child_env))
> AttributeError: 'dict' object has no attribute 'get_env'

becm/meson,jpakkane/meson,becm/meson,jpakkane/meson,trhd/meson,becm/meson,trhd/meson,jpakkane/meson,pexip/meson,becm/meson,ernestask/meson,ernestask/meson,jeandet/meson,jpakkane/meson,pexip/meson,mesonbuild/meson,mesonbuild/meson,ernestask/meson,trhd/meson,QuLogic/meson,thiblahute/meson,mesonbuild/meson,trhd/meson,mesonbuild/meson,jeandet/meson,pexip/meson,ernestask/meson,mesonbuild/meson,pexip/meson,ernestask/meson,trhd/meson,MathieuDuponchelle/meson,jpakkane/meson,MathieuDuponchelle/meson,jeandet/meson,QuLogic/meson,MathieuDuponchelle/meson,MathieuDuponchelle/meson,MathieuDuponchelle/meson,QuLogic/meson,trhd/meson,trhd/meson,jpakkane/meson,thiblahute/meson,QuLogic/meson,thiblahute/meson,QuLogic/meson,becm/meson,thiblahute/meson,MathieuDuponchelle/meson,becm/meson,becm/meson,pexip/meson,pexip/meson,MathieuDuponchelle/meson,jeandet/meson,QuLogic/meson,mesonbuild/meson,MathieuDuponchelle/meson,pexip/meson,thiblahute/meson,ernestask/meson,becm/meson,trhd/meson,pexip/meson,QuLogic/meson,thiblahute/meson,becm/meson,mesonbuild/meson,QuLogic/meson,MathieuDuponchelle/meson,becm/meson,jeandet/meson,mesonbuild/meson,thiblahute/meson,jeandet/meson,thiblahute/meson,ernestask/meson,jpakkane/meson,jeandet/meson,thiblahute/meson,mesonbuild/meson,ernestask/meson,ernestask/meson,jeandet/meson,jeandet/meson,QuLogic/meson,mesonbuild/meson,jpakkane/meson,trhd/meson,pexip/meson,jpakkane/meson,pexip/meson