Alter distclean rule to prevent recursing into the same directory twice

Commit f87e0c03 removed the "system" directory from the TESTDIRS
variable in bin/tests/Makefile.in in an attempt to fix "make distclean"
which was broken since commit 0d784de1.  However, this change
prevented any system tests from being run when "make test" is invoked.

We now put it back into both SUBDIRS and TESTDIRS, but with a modified
rule to check for the existence of a Makefile in each subdirectory before
trying to run make there. This prevents "make distclean" from trying to
run again in a directory where it's already been run.
7 jobs for 193-fix-variable-definitions-in-bin-tests-Makefile.in in 6 minutes and 31 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Precheck
passed #7545
docker linux
precheck:debian:sid:amd64

00:00:11

 
  Build
passed #7546
docker linux
build:debian:sid:amd64

00:01:35

passed #7547
docker linux
build:debian:sid:i386

00:01:41

 
  Test
passed #7550
docker linux
systemtest:debian:sid:amd64

00:04:37

passed #7551
docker linux
systemtest:debian:sid:i386

00:04:38

passed #7548
docker linux
unittest:debian:sid:amd64

00:00:46

passed #7549
docker linux
unittest:debian:sid:i386

00:00:47