tests: use (esc) markup for string-escape
This makes test output less ambiguous.
Failing test output will be escaped and marked up if necessary. A Python
string-escape compatible encoding is used, but not everything is encoded -
especially not \n and \t and '.
tests: (no-eol) markup for command output without trailing LF
Output chunks without a trailing LF will now work but get (no-eol) appended.
This change mostly moves code around so we can handle that an output line
starts with data from previous command, followed by salt and the next command.