unit-tests: raise level to c++14 to fix FTBFS
Fixes:
/usr/src/googletest/googletest/include/gtest/internal/gtest-port.h:270:2: error: #error C++ versions less than C++14 are not supported.
270 | #error C++ versions less than C++14 are not supported.
| ^~~~~
Thanks: Adrian Bunk bunk@debian.org
Closes: #1041089
Edited by Michael Prokop