[5.0] Add additional tests for mapped_private

[heifner]

Add mapped_private to db_modes_test.sh.

[spoonincode]

mapped_private only works on some platforms so need to skip it on unsupported platforms

[greg7mdp]

mapped_private only works on some platforms so need to skip it on unsupported platforms

There are two features "mapped_private" depends on, the soft-dirty feature, and the copy_on_write (i.e. MAP_PRIVATE) file mapping option. The mapped_private mode would still work if soft-dirty is not available, but copy_on_write is.