Add -g option in zdtm.sh.

Using -g option can just generate binaries of all
tests to run in zdtm. Moreover, appending test name after
-g option can just generate the binary for the test.
Signed-off-by: Yicheng Qin <yichengq@google.com>
Acked-by: Andrew Vagin <avagin@parallels.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>