[-Wunsafe-buffer-usage] Extract the included part in tests to separate header files

Extract included parts in the following tests to separate header files:
- SemaCXX/warn-unsafe-buffer-usage-fixits-parm-span-overload.cpp
- SemaCXX/warn-unsafe-buffer-usage-fixits-parm-span.cpp

Removed the included part in the following tests as it is not useful:
- SemaCXX/warn-unsafe-buffer-usage-warning-unevaluated-context.cpp