Move uitests to their own crate

crates/compiler/uitest/tests/pattern/as/does_not_narrow.txt

@@ -0,0 +1,14 @@
+app "test" provides [main] to "./platform"
+
+input : [A Str, B Str]
+input = A "foo"
+
+drop : a -> {}
+drop = \_ -> {}
+
+main = when input is
+    #       ^^^^^ [A Str, B Str]
+    A _ as a -> drop a
+    #                ^ [A Str, B Str]
+    B _ as b -> drop b
+    #                ^ [A Str, B Str]

crates/compiler/uitest/tests/pattern/as/list.txt

@@ -0,0 +1,10 @@
+app "test" provides [main] to "./platform"
+
+input : List Str
+input = [ "foo", "bar" ]
+
+main = when input is
+    #       ^^^^^ List Str
+    [ _first, .. as rest ] -> 1 + List.len rest
+    #                                      ^^^^ List Str
+    [] -> 0

crates/compiler/uitest/tests/pattern/as/uses_inferred_type.txt

@@ -0,0 +1,7 @@
+app "test" provides [main] to "./platform"
+
+main = when A "foo" is
+    A _ as a -> a
+    #           ^ [A Str]w_a
+    b -> b
+    #    ^ [A Str]w_a

crates/compiler/uitest/tests/pattern/infer_type_with_underscore_destructure_assignment.txt

@@ -0,0 +1,6 @@
+app "test" provides [main] to "./platform"
+
+Pair x _ = Pair 0 1
+
+main = x
+#      ^ Num w_a