tests/float: Convert "sys.exit()" to "raise SystemExit".

The latter is shorter and simpler because it doesn't require importing the
sys module.

tests/float/math_fun_int.py

@@ -4,8 +4,7 @@ try:
     import math
 except ImportError:
     print("SKIP")
-    import sys
-    sys.exit()
+    raise SystemExit
 
 for fun in (math.ceil, math.floor, math.trunc):
     for x in (-1.6, -0.2, 0, 0.6, 1.4, float('inf'), float('nan')):