make spec infra more ergonomic

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

kaptan-spec/tests/ui/fail_empty_explanation.stderr

@@ -0,0 +1,7 @@
+error: no-spec explanation must not be empty; use no_spec!("reason");
+ --> tests/ui/fail_empty_explanation.rs:3:1
+  |
+3 | no_spec!("   ");
+  | ^^^^^^^^^^^^^^^
+  |
+  = note: this error originates in the macro `no_spec` (in Nightly builds, run with -Z macro-backtrace for more info)