Migrate to trybuild from compiletest

This migrates our UI tests to the recently published `trybuild` crate
which should hopefully be a bit more robust than `compiletest` over
time!

crates/macro/ui-tests/start-function.stderr

@@ -1,14 +1,11 @@
 error: the start function cannot have arguments
-  --> $DIR/start-function.rs:11:13
-   |
-11 | pub fn foo2(x: u32) {}
-   |             ^^^^^^
+ --> $DIR/start-function.rs:7:13
+  |
+7 | pub fn foo2(x: u32) {}
+  |             ^^^^^^
 
 error: the start function cannot have generics
-  --> $DIR/start-function.rs:14:12
+  --> $DIR/start-function.rs:10:12
    |
-14 | pub fn foo3<T>() {}
+10 | pub fn foo3<T>() {}
    |            ^^^
-
-error: aborting due to 2 previous errors
-