finish factoring tests (#17)

* atoms in one test unit

* factor out pointers test

* factor structs into separate test unit

* factor out arrays, flags

* finally, separate into strings and ints

tests/atoms.witx

@@ -0,0 +1,12 @@
+(use "errno.witx")
+
+(module $atoms
+  (@interface func (export "int_float_args")
+    (param $an_int u32)
+    (param $an_float f32)
+    (result $error $errno))
+  (@interface func (export "double_int_return_float")
+    (param $an_int u32)
+    (result $error $errno)
+    (result $doubled_it f32))
+)