Add a sanity check that signbit works

The array-api-strict package disables signbit at runtime when the API version
is set to 2022.12. This happens when the function is called, so the hasattr
check would pass even though the function call fails.

Author: asmeurer

array_api_tests/pytest_helpers.py

@@ -421,6 +421,15 @@ def assert_fill(
         assert xp.all(xp.equal(out, xp.asarray(fill_value, dtype=dtype))), msg
 
 
+def _has_signbit() -> bool:
+    if not hasattr(_xp, "signbit"):
+        return False
+    try:
+        assert _xp.all(_xp.signbit(_xp.asarray(0.0)) == False)
+    except:
+        return False
+    return True
+
 def _real_float_strict_equals(out: Array, expected: Array) -> bool:
     nan_mask = xp.isnan(out)
     if not xp.all(nan_mask == xp.isnan(expected)):
@@ -429,7 +438,7 @@ def _real_float_strict_equals(out: Array, expected: Array) -> bool:
 
     # Test sign of zeroes if xp.signbit() available, otherwise ignore as it's
     # not that big of a deal for the perf costs.
-    if hasattr(_xp, "signbit"):
+    if _has_signbit():
         out_zero_mask = out == 0
         out_sign_mask = _xp.signbit(out)
         out_pos_zero_mask = out_zero_mask & out_sign_mask