test-getgroups.c:stringop-overflow warning on newer GCC

[Bernhard Voelker <mail@bernhard-voelker.de>]

Newer GCC complains about this test:

  $ make test-getgroups.o V=1
  depbase=`echo test-getgroups.o | sed 's|[^/]*$|.deps/&|;s|\.o$||'`;\
  gcc -DHAVE_CONFIG_H -DEXEEXT=\"\" -I. -I..  -DIN_FINDUTILS_GNULIB_TESTS=1 -I. -I. -I.. -I./.. -I../gl/lib -I./.. \
    /gl/lib   -g -O2 -MT test-getgroups.o -MD -MP -MF $depbase.Tpo -c -o test-getgroups.o test-getgroups.c &&\
  mv -f $depbase.Tpo $depbase.Po
  In file included from test-getgroups.c:31:
  test-getgroups.c: In function 'main':
  test-getgroups.c:65:11: warning: argument 1 value -1 is negative [-Wstringop-overflow=]
     65 |   ASSERT (getgroups (-1, NULL) == -1);
        |           ^~~~~~~~~~~~~~~~~~~~
  macros.h:57:13: note: in definition of macro 'ASSERT'
     57 |       if (!(expr))                                                           \
        |             ^~~~
  In file included from ../gl/lib/unistd.h:40,
                   from test-getgroups.c:21:
  /usr/include/unistd.h:701:12: note: in a call to function 'getgroups' declared with attribute 'write_only (2, 1)'
    701 | extern int getgroups (int __size, __gid_t __list[]) __THROW __wur
        |            ^~~~~~~~~

This is the whole declaration from the system's unistd.h:

  $ sed -n 698,702p /usr/include/unistd.h
  /* If SIZE is zero, return the number of supplementary groups
     the calling process is in.  Otherwise, fill in the group IDs
     of its supplementary groups in LIST and return the number written.  */
  extern int getgroups (int __size, __gid_t __list[]) __THROW __wur
      __attr_access ((__write_only__, 2, 1));

I could reproduce on openSUSE:Tumbleweed and Fedora 33,
both having GCC-10.2.1:

  $ gcc --version | sed 1q
  gcc (SUSE Linux) 10.2.1 20201117 [revision 98ba03ffe0b9f37b4916ce6238fad754e00d720b]

  $ gcc --version | sed 1q
  gcc (GCC) 10.2.1 20201125 (Red Hat 10.2.1-9)

Well, the test is explicitly testing the behavior when passing a negative number.
Maybe we should add this to the test?

  # pragma GCC diagnostic ignored "-Wstringop-overflow"

Thanks & have a nice day,
Berny