Flaky test: TestPluginSocketBackwardsCompatible

[thaJeztah]

Description

Test has failed on various PRs;

 === FAIL: e2e/cli-plugins TestPluginSocketBackwardsCompatible/detached/the_main_CLI_exits_after_3_signals (1.11s)
    socket_test.go:111: docker --config /tmp/plugin-test-1548696475 presocket test-no-socket
    socket_test.go:139: assertion failed: 255 (int) != 1 (int)
    socket_test.go:140: assertion failed: expected error to contain "exit status 1", got "exit status 255"
    socket_test.go:145: assertion failed: 
        --- ←
        +++ →
        @@ -1,2 +1,2 @@
        -exit status -1
        +got 3 SIGTERM/SIGINTs, forcefully exiting
         

=== Failed
=== FAIL: e2e/cli-plugins TestPluginSocketBackwardsCompatible/detached/the_main_CLI_exits_after_3_signals (1.11s)
    socket_test.go:110: docker --config /tmp/plugin-test-2009379327 presocket test-no-socket
    socket_test.go:134: assertion failed: expected error to contain "exit status 1", got "exit status 255"