Allowing PERFPOOL to be defined by zfs-test users

Reviewed-by: John Wren Kennedy <john.kennedy@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Kevin Greene <kevin.greene@delphix.com>
Closes #15868 

tests/zfs-tests/include/default.cfg.in

@@ -80,7 +80,7 @@ export TESTPOOL=testpool
 export TESTPOOL1=testpool1
 export TESTPOOL2=testpool2
 export TESTPOOL3=testpool3
-export PERFPOOL=perfpool
+export PERFPOOL=${PERFPOOL:-perfpool}
 
 # some test file system names
 export TESTFS=testfs