Revive dialup system test

Properly template the port number in config files with copy_setports.

The test takes two minutes on my machine which doesn't seem like a
proper justification to exclude it from the test suite, especially
considering we run these tests in parallel nowadays. The resource usage
doesn't seems significantly increased so it shouldn't interfere with
other system tests.

There also exists a precedent for longer running system tests that are
already part of the default system test suite (e.g. serve-stale takes
almost three minutes on the same machine).

bin/tests/system/conf.sh.common

@@ -28,9 +28,8 @@ export LANG=C
 #
 # The "stress" test is not run by default since it creates enough
 # load on the machine to make it unusable to other users.
-# The "dialup" and "dupsigs" tests are also not run by
-# default because they take a very long time to complete.
-#
+# The "dupsigs" test is not run by default because it takes
+# a very long time to complete.
 
 #
 # These tests can use ports assigned by the caller (other than 5300