Java tests do not run if the project's path contains "@"

Created by: bocon13

When the project's path contains a directory that contains an "@", Buck fails to run Java tests.

Error: Unable to locate junit on the classpath. Please add as a test dependency.

The root cause is that FileClassPathRunner splits the classfile path prematurely -- on the "@" in the project path rather than on the "@" used as a sentinel -- so the classfile cannot be found and is ignored.

For example: file:/tmp/buck@2/@/tmp/buck@2/buck-out/gen/test/classpath-file becomes 2/@/tmp/buck@2/buck-out/gen/test/classpath-file

One example where this issue comes up in practice is that Jenkins will create workspace directories with "@" for concurrent builds on the same machine (e.g. workspace@2).