Fix 'failing/85 gtest dependency with version' test to fail on Arch

If gtest is patched to have a pkg-config file, that will report the
version, so force the 'system' method to be used when we are exercising
that an unknown version doesn't satisfy any version constraint.

test cases/failing/85 gtest dependency with version/meson.build

@@ -1,3 +1,3 @@
 project('gtest dependency with version', ['c', 'cpp'])
 # discovering gtest version is not yet implemented
-dep = dependency('gtest', version: '>0')
+dep = dependency('gtest', method: 'system', version: '>0')