flush line-clearing on progress stop before post-invoke

All other interactions with std::cout are flushed directly, just in the
stop case we hadn't done it – no problem expect if there is still output
coming after apt is done like in the case of a post-invoke script
producing output.

Closes: 793672

apt-pkg/install-progress.cc

@@ -333,6 +333,7 @@ void PackageManagerFancy::Stop()
       // override the progress line (sledgehammer)
       static const char* clear_screen_below_cursor = "\033[J";
       std::cout << clear_screen_below_cursor;
+      std::flush(std::cout);
    }
    child_pty = -1;
 }

test/integration/test-failing-maintainer-scripts

@@ -84,5 +84,8 @@ testdpkginstalled failure-postrm
 testmyfailure aptget purge failure-postrm -y
 
 # FIXME: test with output going to a PTY as it usually does
-#cp -a rootdir/var/lib/dpkg/status.backup rootdir/var/lib/dpkg/status
-#aptget install failure-preinst -y
+if false; then
+	cp -a rootdir/var/lib/dpkg/status.backup rootdir/var/lib/dpkg/status
+	apt install dependee -y || true
+	apt install failure-postinst -y || true
+fi