compel/tests: add sections table & string section tests

Now it has 4 new tests:
ok 4 - section table start oob (64-bit ELF)
ok 5 - too many sections in table (64-bit ELF)
ok 6 - strings section's header oob of section table (64-bit ELF)
ok 7 - strings section oob (64-bit ELF)

I.e, if we forget to test string section's header oob with the next diff:
>--- a/compel/handle-elf.c
>+++ b/compel/handle-elf.c
>@@ -122,7 +122,7 @@ static const char *get_strings_section(Ehdr_t *hdr, uintptr_t mem,
>                pr_err("String section @%#zx size %#lx is out of [%#zx, %#zx)\n",
>                        addr, (unsigned long)secstrings_hdr->sh_size,
>                        mem, mem + size);
>-               return NULL;
>+               return (void*)addr;
>        }
>
>        return (void*)addr;

It will yell with:
ok 1 - zero ELF header (64-bit ELF)
...
not ok 6 - strings section's header oob of section table (64-bit ELF), expected -4 but ret is -1
...
not ok 12 - strings section's header oob of section table (32-bit ELF), expected -4 but ret is -1

Should be more useful when I add relocations tests after all.
(but this seems for me useful too).

Cc: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Dmitry Safonov <dsafonov@virtuozzo.com>
Reviewed-by: Cyrill Gorcunov <gorcunov@gmail.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>