cranelift-filetests: tell when a run test is skipped (fixes #1558);

cranelift/filetests/src/subtest.rs

@@ -26,6 +26,9 @@ pub struct Context<'a> {
     /// Target ISA to test against. Only guaranteed to be present for sub-tests whose `needs_isa`
     /// method returned `true`. For other sub-tests, this is set if the test file has a unique ISA.
     pub isa: Option<&'a dyn TargetIsa>,
+
+    /// Full path to the file containing the test.
+    pub file_path: &'a str,
 }
 
 impl<'a> Context<'a> {