m4/dovecot.m4: rename valgrind output file to hide it from certain tests

m4/dovecot.m4: rename valgrind output file to hide it from certain tests

Some lib-http tests use readdir() to create test cases, and sometimes
object to the contents of test.out.$$. However, as lib-http is also
sensitive to certain characters in the filenames of the files it uses,
we can hide the valgrind output by chosing its filename to exclude it
from the lib_http tests. Non-initial '~' is not known to cause any issues
for either the shell or any known operating system, so is the safest of
lib-http's 'dodgy' characters to use as the separator.

Signed-off-by: Phil Carmody <phil@dovecot.fi>