A missing run scratch dir is reported as the test file failing to source #1163

Problem

load_test_files sources each file as:

source "$test_file" 2>"$_BASHUNIT_RUN_OUTPUT_DIR/source_err"

If the run's scratch directory is missing, the redirect fails. Bash reports
that as the command failing, so the runner sees exit 1 with nothing written to
the capture file and reports:

✗ Error: Source
    Failed to source 'b_test.sh' (exit 1, 52 bytes, no stderr)

against a file that is complete and valid. The message names the test file,
which is the one thing that is not at fault — it sent three separate
investigations at the fixture in #1137.

Fix

Restore the directory before sourcing, and fall back to /dev/null if even
that is refused. Losing one file's stderr capture is worth far less than
failing the file for a reason that is not its own.

Scope

This is a robustness fix, not a fix for #1137. I could not reproduce that
flake in twelve isolated Bash 3.0 runs (six with this change, six without), so
whether this mechanism is what hits CI there is unproven. Filed separately for
that reason.