On z/OS, the following error message is not matched correctly in lit tests.
```
EDC5129I No such file or directory.
```
This patch uses a lit config substitution to check for platform specific error messages.
Reviewed By: muiez, jhenderson
Differential Revision: https://reviews.llvm.org/D95246
The internal shell was already threading around a 'cwd' parameter. We
just have to make it mutable so that we can update it as the test script
executes.
If the shell ever grows support for environment variable substitution,
we could also implement support for export.
llvm-svn: 231017