py/objfloat: Fix abs(-0.0) so it returns 0.0.

Nan and inf (signed and unsigned) are also handled correctly by using
signbit (they were also handled correctly with "val<0", but that didn't
handle -0.0 correctly).  A test case is added for this behaviour.

py/objfloat.c

@@ -161,8 +161,7 @@ STATIC mp_obj_t float_unary_op(mp_unary_op_t op, mp_obj_t o_in) {
         case MP_UNARY_OP_POSITIVE: return o_in;
         case MP_UNARY_OP_NEGATIVE: return mp_obj_new_float(-val);
         case MP_UNARY_OP_ABS: {
-            // TODO check for NaN etc
-            if (val < 0) {
+            if (signbit(val)) {
                 return mp_obj_new_float(-val);
             } else {
                 return o_in;

tests/float/builtin_float_abs.py

@@ -0,0 +1,13 @@
+# test builtin abs function with float args
+
+for val in (
+    '1.0',
+    '-1.0',
+    '0.0',
+    '-0.0',
+    'nan',
+    '-nan',
+    'inf',
+    '-inf',
+    ):
+    print(val, abs(float(val)))