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/ints.witx

@@ -0,0 +1,18 @@
+(use "errno.witx")
+
+(typename $cookie
+  (int u64
+    (const $start 0)))
+
+(typename $bool
+  (enum u8
+    $false
+    $true))
+
+(module $ints
+  (@interface func (export "cookie_cutter")
+    (param $init_cookie $cookie)
+    (result $error $errno)
+    (result $is_start $bool)
+  )
+)