[gfx][test] Allow gfx_unittests and escher_unittests to launch subprocesses

This is required to allow a roll of googletest that enables death tests.
Without this, the death tests fail to (re-)launch themselves and so fail
unexpectedly.

INTK-682 #comment [gfx][test] Allow gfx_unittests and escher_unittests to launch subprocesses

Test: Manual with googletest roll
Change-Id: Ia392694d04ac4edafe5d54271a0c94fea251770c
2 files changed