hwdb-test: properly print error output from systemd-hwdb update

hwdb-test: properly print error output from systemd-hwdb update

Because -e was set, we'd exit immediately when systemd-hwdb failed and the
captured error output was never printed. We want to test two things:
that 0 is returned, and that nothing is printed to stderr (to catch
syntax errors in the hwdb).

v2: stop capturing stderr to a variable
v3: capture stderr to a variable and test both error types separately