tests: robustify -k test

Split stack-fcall.c into several compilation units so that intermediate
function calls would not be optimized out by compiler.

* tests/stack-fcall.c: Move intermediate functions to ...
* tests/stack-fcall-*.c: ... new files.
* tests/Makefile.am (stack_fcall_SOURCES): Add stack-fcall-*.c.
6 files changed