[OE-core] Extraneous ${libdir}/${PN}/ptest in ptest-using packages

[Phil Blundell]

I happened to notice today that I'm getting /lib/lttng-tools/ptest/
installed in my rootfs, which I don't especially want.

What's happening here is that ptest.bbclass (since
25885d3b4f679c33a514d858bc20b0e21aa63721) does:

do_install_ptest_base[cleandirs] = "${D}${PTEST_PATH}"

and this causes ${D}${PTEST_PATH} to be created irrespective of the
setting of PTEST_ENABLED, which is not especially desirable.  Can this
be moved to some place where it only takes effect for ptest-enabled
builds?

p.