first pass at splitting out a test, making ctx/errno reusable

tests/trivial.witx

@@ -0,0 +1,8 @@
+(use "errno.witx")
+
+(module $trivial
+  (@interface func (export "int_float_args")
+    (param $an_int u32)
+    (param $an_float f32)
+    (result $error $errno))
+)