[test] Disable TestCustomShell on Linux
[test] Disable TestCustomShell on Linux

ShellExpandArguments is unimplemented on Linux. I need to come up with
another way to test this on Linux.

git-svn-id: 4c4cc70b1ef44ba2b7963015e681894188cea27e@373662 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/lldb,llvm-mirror/lldb,llvm-mirror/lldb,llvm-mirror/lldb,llvm-mirror/lldb