component/nxdtest: v↑ (properly cancel on SIGTERM/SIGINT)

This upgrade teaches nxdtest to properly cancel test run and test job
when it receives SIGTERM or SIGINT.

See nxdtest@6f75fa90...56e52da6 for details.

/reviewed-by @jerome
/reviewed-on !1121

component/nxdtest/buildout.cfg

@@ -31,6 +31,6 @@ depends = ${python-prctl:egg}
 [nxdtest-repository]
 recipe  = slapos.recipe.build:gitclone
 repository = https://lab.nexedi.com/nexedi/nxdtest.git
-revision = 6f75fa90
+revision = 56e52da6
 location = ${buildout:parts-directory}/nxdtest
 git-executable = ${git:location}/bin/git