tests/test-subr.sh: Put test_dir under /var/tmp.

tests/test-subr.sh: Put test_dir under /var/tmp.

Every individual test in elfutils involves a temporary directory.
Previous version of this script put that directory under the build
tree.  That's OK if it's a local disk, but if it's on NFS, then some
tests - run-large-elf-file.sh, several run-debuginfod-*.sh - take long
enough to run to fail tests intermittently.

This patch moves the temp_dir under ${TMPDIR-/var/tmp/}, so it
operates at local disk speed rather than whatever-build-filesystem
speed.  Individual test scripts are all unaffected.  (One could
consider /tmp instead, which is a RAM disk on modern systems, except
that some of the elfutils tests produce GB-sized temporary files.
That's probably too big for RAM.)

Signed-off-by: Frank Ch. Eigler <fche@redhat.com>