waf_unit_test: prevent error when dumping run to script and environment contains UTF-8 chars
Since by default latin-1 is used with Python3 when the decode is called if UTF-8 chars are present in the environment it will generate an traceback. This PR puts UTF-8 as default encoding (as in other cases through the code) to prevent this.
To reproduce the problem:
cd demos/unit_test export UNICODE_VAR=✘ waf configure waf --dump-test-scripts