test-capability: silence gcc warning

test-capability: silence gcc warning

[2/3] Compiling C object test-capability.p/src_test_test-capability.c.o
../src/test/test-capability.c: In function ‘main’:
../src/test/test-capability.c:270:12: warning: ‘run_ambient’ may be used uninitialized in this function [-Wmaybe-uninitialized]
  270 |         if (run_ambient)
      |            ^

gcc-11.0.1-0.3.fc34.x86_64