REGTESTS: converter: add a test for the when() and debug() converters

REGTESTS: converter: add a test for the when() and debug() converters

Neither converter was covered. when() only lets its input through when
the condition is met, so the test builds a string made of one bracketed
field per condition and compares it for three kinds of transactions: one
forwarded to a healthy server, one whose server cannot be reached, and
one answered by the stats applet. This covers the error, normal,
forwarded, processed, toapplet, stopping and acl conditions as well as
the negated form.

debug() is checked to be transparent for the sample it is inserted in,
and its copy is read back from the default sink with "show events".