fs-tests: integck: add debug compilation target

fs-tests: integck: add debug compilation target

I often need to use gdb to debug integck, or sometimes even attach
already running integck. But when it is compiled with optimizations, it
is nearly impossible to use gdb. Introduce 'debug' target to compile
integck without optimizations.

Additionally, to make the stack backtraces work, add a dirty hack to
integck.c to remove all the static keywords - this turns static
functions to non-static and makes them appear in stack backtraces.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>