selftests: bpf: break up test_progs - preparations

Add new prog_tests directory where tests are supposed to land.
Each prog_tests/<filename>.c is expected to have a global function
with signature 'void test_<filename>(void)'. Makefile automatically
generates prog_tests/tests.h file with entry for each prog_tests file:

	#ifdef DECLARE
	extern void test_<filename>(void);
	...
	#endif

	#ifdef CALL
	test_<filename>();
	...
	#endif

prog_tests/tests.h is included in test_progs.c in two places with
appropriate defines. This scheme allows us to move each function with
a separate patch without breaking anything.

Compared to the recent verifier split, each separate file here is
a compilation unit and test_progs.[ch] is now used as a place to put
some common routines that might be used by multiple tests.

Signed-off-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>