openssh: Document skipped test dependency

In minimal images the agent-ptrace test is skipped unless gdb is
installed which explains the difference in test counts. We don't
want a build dependency on gdb and the test isn't critical so just
document the dependency.

(From OE-Core rev: d3f29e6e52367e124e3f543f970038c0332ad3e1)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/recipes-connectivity/openssh/openssh_8.0p1.bb

@@ -145,6 +145,7 @@ FILES_${PN}-keygen = "${bindir}/ssh-keygen"
 RDEPENDS_${PN} += "${PN}-scp ${PN}-ssh ${PN}-sshd ${PN}-keygen"
 RDEPENDS_${PN}-sshd += "${PN}-keygen ${@bb.utils.contains('DISTRO_FEATURES', 'pam', 'pam-plugin-keyinit pam-plugin-loginuid', '', d)}"
 RRECOMMENDS_${PN}-sshd_append_class-target = " rng-tools"
+# gdb would make attach-ptrace test pass rather than skip but not worth the build dependencies
 RDEPENDS_${PN}-ptest += "${PN}-sftp ${PN}-misc ${PN}-sftp-server make sed sudo coreutils"
 
 RPROVIDES_${PN}-ssh = "ssh"