tests: Honor TEST_SKIP_CLEANUP like ostree

We seem to have substantial infrastructure drift here between the two, and I was
*really* confused why `TEST_SKIP_CLEANUP` wasn't being honored. Turned out in
rpm-ostree we have tmpdir code in two places, not entirely sure why. Punting
full cleanup.

Closes: #566
Approved by: jlebon

buildutil/tap-test

@@ -6,7 +6,13 @@ srcd=$(cd $(dirname $1) && pwd)
 bn=$(basename $1)
 tempdir=$(mktemp -d)
 function cleanup () {
-    rm "${tempdir}" -rf
+    if test -z "${TEST_SKIP_CLEANUP:-}"; then
+	      if test -f ${test_tmpdir}/.test; then
+            rm "${tempdir}" -rf
+        fi
+    else
+	      echo "Skipping cleanup of ${test_tmpdir}"
+    fi
 }
 trap cleanup EXIT
 cd ${tempdir}

tests/common/libtest.sh

@@ -54,6 +54,7 @@ if test -n "${UNINSTALLEDTESTS:-}"; then
 fi
 
 test_tmpdir=$(pwd)
+echo "Using tmpdir ${test_tmpdir}"
 
 export G_DEBUG=fatal-warnings