test: Add two tests for pre-dump action

The first is similar to existing snapshot test -- it calls
several pre-dumps on memory-changing zdtm test, then dumps
and check restore goes OK.

The second test is aimed at measuring frozen-time -- it
takes zdtm test with big mappings, dumps one, then start the
test again and calls one pre-dump plus dump. The sum of
frozen times in the 2nd dump (well, pre-dump + dump) should
be much smaller then the one of the 1st "just dump".

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>