Replace usage of GTEST_DISALLOW_ASSIGN_
Fixes the following error:
/src/tests/framework/gmock_fixes.h:104:3: error: ISO C++ forbids declaration of 'GTEST_DISALLOW_ASSIGN_' with no type [-fpermissive]
104 | GTEST_DISALLOW_ASSIGN_(ActionResultHolder);
| ^~~~~~~~~~~~~~~~~~~~~~
Similar to https://github.com/MirServer/mir/pull/2307