Move the functional test temporary directory under the build directory

Summary:
The functional tests temporary directoy (containing the logs in case of
a test failure) is currently created using the python tempfile
library. This can cause issues on some systems where /tmp mounted on
RAM and is not large enough to handle the large files produced during
the tests. This issue has been raised in D4927.

This diff moves the temp directory to <build_dir>/test/functional
which should avoid the issue. It is always possible to change this
behavior by passing --tmpdirprefix on the command line.

Test Plan:

1. The test doesn't matter but feature_help runs fast ./test/functional/test_runner.py feature_help

Check the temporary directory is created at the expected location.
Note: since the temporary directory is cleaned upon success, you should
look at stdout to check this.

mkdir -p /tmp/test
./test/functional/test_runner.py --tmpdirprefix=/tmp/test feature_help

Check the temporary directory is created under /tmp/test

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, deadalnix, jasonbcox

Subscribers: markblundeberg

Differential Revision: https://reviews.bitcoinabc.org/D4939