[validation] Fix line-directive validation test

This was disabled due to:
```
Unable to decode the command from the command line:
UnicodeEncodeError: 'utf-8' codec can't encode characters in position 13-20: surrogates not allowed
```

Note that this error isn't in `line-directive`, but in Python decoding
surrogates from the command line. Not sure why that's not allowed, but
printing from within a script works so change to that.

We already set `PYTHONIOENCODING=UTF8` in the environment in
`test/lit.cfg`, which makes sure stdout is UTF8 as opposed to ASCII.

Resolves rdar://92613094.

Author: bnbarham

validation-test/Python/line-directive.swift

@@ -1,4 +1,8 @@
-// REQUIRES: rdar92613094
+// RUN: %empty-directory(%t)
+// RUN: split-file %s %t
 
 // RUN: %{python} %utils/line-directive
-// RUN: %{python} %utils/line-directive -- %{python} -c "print('你好')"
+// RUN: %{python} %utils/line-directive -- %{python} %t/unicode.py
+
+//--- unicode.py
+print('你好')