test: remove a new line from containerID for correct test execution

[haytok]

When running TestContainerRmIptables with verbose output, the following error can be observed:

$ sudo go test -run TestContainerRmIptables -v

...

=== CONT  TestContainerRmIptables/Test_iptables_rules_are_cleared_after_container_deletion
    container_remove_linux_test.go:135:

...

    container_remove_linux_test.go:49:

        +------------------------------------------------------------------------------------------------------------+
        | ➡️      | ⚙️ /usr/local/bin/nerdctl rm -f fd4fc99e04476d7f673133a06338933aadc547d4eef883f469464d7d31467504 |
        |         |                                                                                                  |
        +------------------------------------------------------------------------------------------------------------+
        |         | 🟠 time="2026-03-23T00:00:53+09:00" level=error msg="1 errors:\nfilters: parse error: [labels.\" |
        |         | nerdctl/name\"==fd4fc99e04476d7f673133a06338933aadc547d4eef883f469464d7d31467504 >|\n|< ]: unexp |
        |         | ected input: \n: invalid argument: invalid argument"                                             |

...

    container_remove_linux_test.go:135:

        <<<<<<<<<<<<<<<<<<<<
                🖊️ Inspecting output (does not contain)
                👀 testing:             `-P PREROUTING ACCEPT
        -P INPUT ACCEPT

...

        -A POSTROUTING -s 10.4.0.148/32 -m comment --comment "name: \"bridge\" id: \"nerdctl-test-fd4fc99e04476d7f673133a06338933aadc547d4eef883f469464d7d31467504\"" -j CNI-d4bd64738075587aa8d84afc

...

        -A CNI-HOSTPORT-DNAT -p tcp -m comment --comment "dnat name: \"bridge\" id: \"nerdctl-test-fd4fc99e04476d7f673133a06338933aadc547d4eef883f469464d7d31467504\"" -m multiport --dports 5000 -j CNI-DN-d4bd64738075587aa8d84
...
        -A CNI-d4bd64738075587aa8d84afc -d 10.4.0.0/24 -m comment --comment "name: \"bridge\" id: \"nerdctl-test-fd4fc99e04476d7f673133a06338933aadc547d4eef883f469464d7d31467504\"" -j ACCEPT
        -A CNI-d4bd64738075587aa8d84afc ! -d 224.0.0.0/4 -m comment --comment "name: \"bridge\" id: \"nerdctl-test-fd4fc99e04476d7f673133a06338933aadc547d4eef883f469464d7d31467504\"" -j MASQUERADE

...

        -P POSTROUTING ACCEPT
        `
                ✅️ does verify:         ! ~= `fd4fc99e04476d7f673133a06338933aadc547d4eef883f469464d7d31467504
        `
        >>>>>>>>>>>>>>>>>>>>

...

=== NAME  TestContainerRmIptables
    container_remove_linux_test.go:135:

        +============================================================================================================+
        | 🧽      | "TestContainerRmIptables": post-cleanup                                                          |
        +============================================================================================================+

--- PASS: TestContainerRmIptables (0.00s)
    --- PASS: TestContainerRmIptables/Test_iptables_rules_are_cleared_after_container_deletion (2.93s)
PASS
ok      github.com/containerd/nerdctl/v2/cmd/nerdctl/container  2.936s

The error unexpected input: \n: invalid argument: invalid argument indicates that the container is not being removed correctly.

Additionally, since the container ID remains in the iptables output, it is clear that expect.DoesNotContain(containerID) passes for the wrong reason, and the test is not properly validating the iptables cleanup behavior after container removal.

This commit removes the new line from the container ID using strings.TrimSpace.

[haytok]

HI, @AkihiroSuda

When you have time, could you re-try CIs ?

[AkihiroSuda]

Thanks