logger: add -DTEST_LOGGER

logger: add -DTEST_LOGGER

"make test_logger" now compiles logger(1) test program
to overwrite system datetime stuff, hostname and PID, for example:

   export TZ=GMT
   export LOGGER_TEST_TIMEOFDAY=1234567890.123456
   export LOGGER_TEST_HOSTNAME=foo
   export LOGGER_TEST_GETPID=123

   ./test_logger --rfc5424 --no-act --stderr -i --tag MyTag mesg
   <13>1 2009-02-13T23:31:30.123456+00:00 foo MyTag 123 - [timeQuality tzKnown="1" isSynced="0"] mesg

if the LOGGER_TEST_* variables are not specified then default to
standard logger(1) behavior.

Note that it would be possible to use for example "unshare --utc" to
make hostname stable and portable, but LOGGER_TEST_* variables allow
to keep the tests less complex.

Signed-off-by: Karel Zak <kzak@redhat.com>