improve kaptan-spec as a build library and move it to libs dir

Signed-off-by: Onur Özkan <work@onurozkan.dev>

libs/kaptan-spec/tests/spec_macro.rs

@@ -0,0 +1,14 @@
+#[test]
+fn accepts_no_spec_reason() {
+    let tests = trybuild::TestCases::new();
+    tests.pass("tests/ui/no_spec.rs");
+}
+
+#[test]
+fn rejects_invalid_no_spec_invocations() {
+    let tests = trybuild::TestCases::new();
+
+    tests.compile_fail("tests/ui/fail_missing_argument.rs");
+    tests.compile_fail("tests/ui/fail_non_string_argument.rs");
+    tests.compile_fail("tests/ui/fail_empty_explanation.rs");
+}