Split argument list, satisfy shellcheck SC2086

Split argument list, satisfy shellcheck SC2086

Split the arguments for ${TEST_RUNNER} across multiple lines for
clarity. Also added quotes in the message to match the invoked command.

Unquoted variables in argument lists are subject to splitting. In this
particular case we can't quote the variable because it is an optional
argument. Use the method suggested in the description linked below,
instead.

The technique is to use an unquoted variable with an alternate value.

https://github.com/koalaman/shellcheck/wiki/SC2086

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Giuseppe Di Natale <guss80@gmail.com>
Reviewed-by: John Kennedy <john.kennedy@delphix.com>
Signed-off-by: Ryan Moeller <ryan@ixsystems.com>
Closes #9212