refactor: replace SubTests with Setup+helper in TestMultiPlatformRun

SubTests run via t.Parallel() in Tigron, which caused concurrent docker
runs across platforms to fail in the in-host/docker CI job. The original
test iterated platforms sequentially; use the existing assertMultiPlatformRun
helper inside Setup to preserve that behavior.

Signed-off-by: Ogulcan Aydogan <ogulcanaydogan@hotmail.com>

Author: ogulcanaydogan

cmd/nerdctl/container/multi_platform_linux_test.go

@@ -77,20 +77,8 @@ func TestMultiPlatformRun(t *testing.T) {
 
 	testCase.Require = requireMultiPlatformExec
 
-	testCasePlatforms := map[string]string{
-		"amd64":        "x86_64",
-		"arm64":        "aarch64",
-		"arm":          "armv7l",
-		"linux/arm":    "armv7l",
-		"linux/arm/v7": "armv7l",
-	}
-	for plat, expectedUnameM := range testCasePlatforms {
-		p, e := plat, expectedUnameM
-		testCase.SubTests = append(testCase.SubTests, &test.Case{
-			Description: p,
-			Command:     test.Command("run", "--rm", "--platform="+p, testutil.AlpineImage, "uname", "-m"),
-			Expected:    test.Expects(expect.ExitCodeSuccess, nil, expect.Equals(e+"\n")),
-		})
+	testCase.Setup = func(_ test.Data, helpers test.Helpers) {
+		assertMultiPlatformRun(helpers, testutil.AlpineImage)
 	}
 
 	testCase.Run(t)