selftests: harness: Rename is_signed_type() to avoid collision with overflow.h

Rename is_signed_type() to is_signed_var() to avoid colliding with a macro
of the same name defined by tools' linux/overflow.h.  This fixes warnings
(and presumably potential test failures) in tests that utilize the
selftests harness and happen to (indirectly) include overflow.h.

  In file included from tools/include/linux/bits.h:34,
                   from tools/include/linux/bitops.h:14,
                   from tools/include/linux/hashtable.h:13,
                   from include/kvm_util.h:11,
                   from x86/userspace_msr_exit_test.c:11:
  tools/include/linux/overflow.h:31:9: error: "is_signed_type" redefined [-Werror]
     31 | #define is_signed_type(type)       (((type)(-1)) < (type)1)
        |         ^~~~~~~~~~~~~~
  In file included from include/kvm_test_harness.h:11,
                   from x86/userspace_msr_exit_test.c:9:
  ../kselftest_harness.h:754:9: note: this is the location of the previous definition
    754 | #define is_signed_type(var)       (!!(((__typeof__(var))(-1)) < (__typeof__(var))1))
        |         ^~~~~~~~~~~~~~

Use a separate definition, at least for now, as many selftests build
without tools/include in their include path.

Fixes: fc92099 ("tools headers: Synchronize linux/bits.h with the kernel sources")
Cc: Vincent Mailhol <mailhol.vincent@wanadoo.fr>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20250624231930.583689-1-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: sean-jc

tools/testing/selftests/kselftest_harness.h

@@ -751,15 +751,15 @@
 	for (; _metadata->trigger; _metadata->trigger = \
 			__bail(_assert, _metadata))
 
-#define is_signed_type(var)       (!!(((__typeof__(var))(-1)) < (__typeof__(var))1))
+#define is_signed_var(var)	(!!(((__typeof__(var))(-1)) < (__typeof__(var))1))
 
 #define __EXPECT(_expected, _expected_str, _seen, _seen_str, _t, _assert) do { \
 	/* Avoid multiple evaluation of the cases */ \
 	__typeof__(_expected) __exp = (_expected); \
 	__typeof__(_seen) __seen = (_seen); \
 	if (!(__exp _t __seen)) { \
 		/* Report with actual signedness to avoid weird output. */ \
-		switch (is_signed_type(__exp) * 2 + is_signed_type(__seen)) { \
+		switch (is_signed_var(__exp) * 2 + is_signed_var(__seen)) { \
 		case 0: { \
 			uintmax_t __exp_print = (uintmax_t)__exp; \
 			uintmax_t __seen_print = (uintmax_t)__seen; \