selftests/core: fix conflicting types compile error for close_range()

selftests/core: fix conflicting types compile error for close_range()

close_range() test type conflicts with close_range() library call in
x86_64-linux-gnu/bits/unistd_ext.h. Fix it by changing the name to
core_close_range().

gcc -g -I../../../../usr/include/    close_range_test.c  -o ../tools/testing/selftests/core/close_range_test
In file included from close_range_test.c:16:
close_range_test.c:57:6: error: conflicting types for ‘close_range’; have ‘void(struct __test_metadata *)’
   57 | TEST(close_range)
      |      ^~~~~~~~~~~
../kselftest_harness.h:181:21: note: in definition of macro ‘__TEST_IMPL’
  181 |         static void test_name(struct __test_metadata *_metadata); \
      |                     ^~~~~~~~~
close_range_test.c:57:1: note: in expansion of macro ‘TEST’
   57 | TEST(close_range)
      | ^~~~
In file included from /usr/include/unistd.h:1204,
                 from close_range_test.c:13:
/usr/include/x86_64-linux-gnu/bits/unistd_ext.h:56:12: note: previous declaration of ‘close_range’ with type ‘int(unsigned int,  unsigned int,  int)’
   56 | extern int close_range (unsigned int __fd, unsigned int __max_fd,
      |            ^~~~~~~~~~~

Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>