Issue #7151: regrtest would generate a JSON failure if there was output

Issue #7151: regrtest would generate a JSON failure if there was output
to stderr during the test run and it happened to get emitted after the
worker thread emitted the result JSON.  Now we capture stdout and stderr
separately, which avoids that problem.  It also means that _all_ stderr
output is after all stdout output when we print the test results, but
that seems acceptable, since output ordering is not guaranteed anyway.
The patch also moves the emit of the test name into the output block
generated after the test completes.  Otherwise test names and test
output/errors were mixed in the terminal display, making it difficult
to determine which test generated the output.