Indent multiline output in pytest logging

Indent multiline output in pytest logging

When multiline message is logged, indent all but the first line (which
will be preceeded by the LOG_FORMAT). This improves the clarity of logs,
as it's immediately clear which lines are regular log output, and which
ones are multiline debug output.

Adjust the isctest.run.cmd() stdout/stderr logging to this new format.

(cherry picked from commit 23e6b49cc57cb41a0260686366e7ba86cac0ec4a)