ZTS: Replace /var/tmp with $TEST_BASE_DIR

Remove a few hardcoded instances of /var/tmp.  This should use
the $TEST_BASE_DIR in order to allow the ZTS to be optionally
run in an alternate directory using `zfs-tests.sh -d <path>`.

Reviewed-by: Ryan Moeller <ryan@ixsystems.com>
Reviewed-by: John Kennedy <john.kennedy@delphix.com>
Reviewed-by: Igor Kozhukhov <igor@dilos.org>
Reviewed-by: Kjeld Schouten <kjeld@schouten-lebbing.nl>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes openzfs#9775

tests/zfs-tests/tests/functional/cli_root/zfs_send/zfs_send_001_pos.ksh

@@ -75,7 +75,7 @@ log_onexit cleanup
 init_snap=$TESTPOOL/$TESTFS@init_snap
 inc_snap=$TESTPOOL/$TESTFS@inc_snap
 full_bkup=$TEST_BASE_DIR/fullbkup.$$
-inc_bkup=/var/tmp/incbkup.$$
+inc_bkup=$TEST_BASE_DIR/incbkup.$$
 init_data=$TESTDIR/$TESTFILE1
 inc_data=$TESTDIR/$TESTFILE2
 orig_sum=""

tests/zfs-tests/tests/functional/cli_user/zfs_list/zfs_list_007_pos.ksh

@@ -57,9 +57,8 @@ function cleanup
 log_onexit cleanup
 log_assert "'zfs list -d <n>' should get expected output."
 
-mntpnt=/var/tmp
-DEPTH_OUTPUT="$mntpnt/depth_output"
-EXPECT_OUTPUT="$mntpnt/expect_output"
+DEPTH_OUTPUT="$TEST_BASE_DIR/depth_output"
+EXPECT_OUTPUT="$TEST_BASE_DIR/expect_output"
 typeset -i old_val=0
 typeset -i j=0
 typeset -i fs=0