validation: make test_shell Python3 friendly

The custom `shell` module would coerce the byte-string representation
from `subprocess` into a text string representation in the happy path.
However, the same handling was not applied to the error case.  This
would result in a byte-string representation being returned in Python 3
rather than the text string.  Perform the conversion in both cases to
ensure that we can handle the strings without having to do the
conversion.

Author: compnerd

utils/swift_build_support/swift_build_support/shell.py

@@ -137,7 +137,7 @@ def capture(command, stderr=None, env=None, dry_run=None, echo=True,
         return str(out.decode())
     except subprocess.CalledProcessError as e:
         if allow_non_zero_exit:
-            return e.output
+            return str(e.output.decode())
         if optional:
             return None
         _fatal_error(