Remap path refix in the panic message

Otherwise `error-pattern` on the test run stderr can incorrectly match
if e.g. the paths in panic backtrace has a matching substring (like if
we look for `bar` in the error pattern but username is `baron`).

tests/ui/meta/revision-bad.rs

@@ -5,6 +5,7 @@
 //@ revisions: foo bar
 //@ should-fail
 //@ needs-run-enabled
+//@ compile-flags: --remap-path-prefix={{src-base}}=remapped
 //@[foo] error-pattern:bar
 //@[bar] error-pattern:foo