test/failed-doctest-output: Fix normalization

Otherwise we get:

1
2       running 2 tests
-       test $DIR/failed-doctest-output.rs - OtherStruct (line 20) ... FAILED
-       test $DIR/failed-doctest-output.rs - SomeStruct (line 10) ... FAILED
+       test src/rustc-1.38.0-src/$DIR/failed-doctest-output.rs - OtherStruct (line 20) ... FAILED
+       test src/rustc-1.38.0-src/$DIR/failed-doctest-output.rs - SomeStruct (line 10) ... FAILED
5
6       failures:
7

tests/rustdoc-ui/doctest/failed-doctest-output.rs

@@ -7,7 +7,7 @@
 
 // compile-flags:--test --test-args --test-threads=1
 // rustc-env:RUST_BACKTRACE=0
-// normalize-stdout-test: "tests/rustdoc-ui/doctest" -> "$$DIR"
+// normalize-stdout-test: "[[:graph:]]*tests/rustdoc-ui/doctest" -> "$$DIR"
 // normalize-stdout-test "finished in \d+\.\d+s" -> "finished in $$TIME"
 // failure-status: 101