fix race condition in TestRemoveForce

[thaJeztah]

relates to #3519 (comment)

This test uses two subtests that were sharing the same variable.
Subtests run in a goroutine, which could lead to them concurrently
accessing the variable, resulting in a panic:

=== FAIL: cli/command/container TestRemoveForce/without_force (0.00s)
Error: Error: No such container: nosuchcontainer
    --- FAIL: TestRemoveForce/without_force (0.00s)
panic: runtime error: invalid memory address or nil pointer dereference [recovered]
	panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x40393f]
goroutine 190 [running]:
testing.tRunner.func1.2({0xb76380, 0x124c9a0})
	/usr/local/go/src/testing/testing.go:1389 +0x24e
testing.tRunner.func1()
	/usr/local/go/src/testing/testing.go:1392 +0x39f
panic({0xb76380, 0x124c9a0})
	/usr/local/go/src/runtime/panic.go:838 +0x207
sort.StringSlice.Less(...)
	/usr/local/go/src/sort/sort.go:319
sort.insertionSort({0xd87380, 0xc00051b3b0}, 0x0, 0x2)
	/usr/local/go/src/sort/sort.go:40 +0xb1
sort.quickSort({0xd87380, 0xc00051b3b0}, 0x18?, 0xb4f060?, 0xc000540e01?)
	/usr/local/go/src/sort/sort.go:222 +0x171
sort.Sort({0xd87380, 0xc00051b3b0})
	/usr/local/go/src/sort/sort.go:231 +0x53
sort.Strings(...)
	/usr/local/go/src/sort/sort.go:335
github.com/docker/cli/cli/command/container.TestRemoveForce.func2(0xc0005389c0?)
	/go/src/github.com/docker/cli/cli/command/container/rm_test.go:36 +0x125
testing.tRunner(0xc00053e4e0, 0xc00051b140)
	/usr/local/go/src/testing/testing.go:1439 +0x102
created by testing.(*T).Run
	/usr/local/go/src/testing/testing.go:1486 +0x35f
=== FAIL: cli/command/container TestRemoveForce (0.00s)

This patch changes the test to use to separate variables.

Signed-off-by: Sebastiaan van Stijn github@gone.nl

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