`TestCleanupMountsAfterDaemonAndContainerKill` was supposedly written
when the container mounts were visible from the host. Currently they
all live in their own mount namespace and the only visible mount is
the tmpfs one for shareable /dev/shm inside the container (i.e.
/var/lib/docker/containers/<ID>/shm), which will no longer be there
in case of `--default-ipc-mode private` is used, and so the test will
fail. Add a check if any container mounts are visible from the host,
and skip the test if there are none, as there's nothing to check.

`TestCleanupMountsAfterDaemonCrash`: fix in a similar way, keeping
all the other checks it does, and skipping the "mounts gone" check
if there were no mounts visible from the host.

While at it, also fix the tests to use `d.Kill()` in order to not
leave behind a stale `docker.pid` files.

Signed-off-by: Kir Kolyshkin <[email protected]>

I got the following test failure on power:

10:00:56
10:00:56
----------------------------------------------------------------------
10:00:56 FAIL: docker_cli_build_test.go:3521:
DockerSuite.TestBuildNotVerboseFailureRemote
10:00:56
10:00:56 docker_cli_build_test.go:3536:
10:00:56     c.Fatal(fmt.Errorf("Test[%s] expected that quiet stderr and
verbose stdout are equal; quiet [%v], verbose [%v]", name,
quietResult.Stderr(), result.Combined()))
10:00:56 ... Error: Test[quiet_build_wrong_remote] expected that quiet
stderr and verbose stdout are equal; quiet [
10:00:56 unable to prepare context: unable to download remote context
http://something.invalid: Get http://something.invalid: dial tcp: lookup
something.invalid on 172.29.128.11:53: no such host
10:00:56 ], verbose [unable to prepare context: unable to download
remote context http://something.invalid: Get http://something.invalid:
dial tcp: lookup something.invalid on 8.8.8.8:53: no such host
10:00:56 ]
10:00:56
10:00:56
10:00:56
----------------------------------------------------------------------

The reason is, either more than one name server is configured, or
nameserver was reconfigured in the middle of the test run. In any case,
different nameserver IP in an error messages should not be treated
as a failure, so let's strip those out.

Signed-off-by: Kir Kolyshkin <[email protected]>

According to golang/go#5373, go recognizes
(and optimizes for) the following syntax:

```go
for i := range b {
	b[i] = 0
}
```

so let's use it. Limited testing shows ~7.5x speed increase,
compared to the previously used syntax.

Signed-off-by: Kir Kolyshkin <[email protected]>