make UI tests that use --test work on panic=abort targets

by adding `-Zpanic_abort_test`, which is a no-op on panic=unwind targets

fixes #135819

Author: japaric

tests/ui/macros/rfc-2011-nicer-assert-messages/assert-with-custom-errors-does-not-create-unnecessary-code.rs

@@ -1,4 +1,6 @@
-//@ compile-flags: --test
+// -Zpanic_abort_tests makes this test work on panic=abort targets and
+// it's a no-op on panic=unwind targets
+//@ compile-flags: --test -Zpanic_abort_tests
 //@ run-pass
 
 #![feature(core_intrinsics, generic_assert)]

tests/ui/macros/rfc-2011-nicer-assert-messages/feature-gate-generic_assert.rs

@@ -1,4 +1,6 @@
-//@ compile-flags: --test
+// -Zpanic_abort_tests makes this test work on panic=abort targets and
+// it's a no-op on panic=unwind targets
+//@ compile-flags: --test -Zpanic_abort_tests
 // ignore-tidy-linelength
 //@ run-pass