Filenames should be escaped in test reproduction instructions

[smarter]

When a test fail, the logfile may contain a message like:

the test can be reproduced by running:
./bin/dotc ...
compiler/../library/src/scala/compat/java8/JFunction0$mcB$sp.java

The filename contains a $ that needs to be escaped to be able to run the given command in a shell

[smarter]

The simplest solution would be to put single quotes around each filename.