test_runner: Readable output if create_cache.py fails

Summary:

Without this change, create_cache.py process output is shown as a byte()
object
with \n escapes in a single line that is hard to read.

Backport of core PR12227
https://github.com/bitcoin/bitcoin/pull/12227/files

Test Plan:
Add the following line:

assert False

to test/functional/create_cache_py just after def run_test(self):.

./test/functional/test_runner.py

The output should be correctly formatted, with multiline text on
multiple lines.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D3457