Replace trybuild tests with some local ones

As it turns out, rustc's "trait unimplemented" error messages have not
stayed perfectly stable across the last 10 versions. This is likely an
issue that would keep happening with trybuild which would make it
somewhat unsuitable.

With some thinking I was able to get the same test coverage without
requiring a compile error.

serde_with/tests/schemars_0_8.rs

@@ -92,11 +92,31 @@ fn schemars_basic() {
     assert_eq!(value, expected);
 }
 
-#[test]
-fn trybuild() {
-    let t = trybuild::TestCases::new();
-    t.pass("tests/trybuild/schemars_0_8/pass.conditional_derive_enabled.rs");
-    t.compile_fail("tests/trybuild/schemars_0_8/fail.conditional_derive_disabled.rs");
+mod derive {
+    use super::*;
+
+    #[serde_with::serde_as]
+    #[derive(Serialize)]
+    #[cfg_attr(all(), derive(JsonSchema))]
+    struct Enabled {
+        #[serde_as(as = "DisplayFromStr")]
+        field: u32,
+    }
+
+    #[serde_with::serde_as]
+    #[derive(Serialize)]
+    #[cfg_attr(any(), derive(JsonSchema))]
+    struct Disabled {
+        // If we are incorrectly adding `#[schemars(with = ...)]` attributes
+        // then we should get an error on this field.
+        #[serde_as(as = "DisplayFromStr")]
+        field: u32,
+    }
+
+    #[test]
+    fn test_enabled_has_correct_schema() {
+        check_valid_json_schema(&Enabled { field: 77 });
+    }
 }
 
 mod array {