tests: allow specifying tmpdir

Allow developers to override the default /var/tmp dir, which e.g. might
be on overlayfs and thus produces reduced coverage.

Closes: #1207
Approved by: cgwalters

buildutil/tap-test

@@ -8,7 +8,8 @@
 
 srcd=$(cd $(dirname $1) && pwd)
 bn=$(basename $1)
-tempdir=$(mktemp -d /var/tmp/tap-test.XXXXXX)
+TEST_TMPDIR=${TEST_TMPDIR:-/var/tmp}
+tempdir=$(mktemp -d $TEST_TMPDIR/tap-test.XXXXXX)
 touch ${tempdir}/.testtmp
 function cleanup () {
     if test -f ${tempdir}/.testtmp; then