platforms: fill default arm variant when parse platform specifier

arm has been supported, but something is missing, causes test failure

    --- FAIL: TestParseSelector/linux (0.00s)
        platforms_test.go:292: arm support not fully implemented: not implemented
    --- FAIL: TestParseSelector/macOS (0.00s)
        platforms_test.go:292: arm support not fully implemented: not implemented

Signed-off-by: Shengjing Zhu <zhsj@debian.org>

platforms/platforms_test.go

@@ -26,10 +26,15 @@ import (
 
 func TestParseSelector(t *testing.T) {
 	var (
-		defaultOS   = runtime.GOOS
-		defaultArch = runtime.GOARCH
+		defaultOS      = runtime.GOOS
+		defaultArch    = runtime.GOARCH
+		defaultVariant = ""
 	)
 
+	if defaultArch == "arm" && cpuVariant != "v7" {
+		defaultVariant = cpuVariant
+	}
+
 	for _, testcase := range []struct {
 		skip      bool
 		input     string
@@ -255,8 +260,9 @@ func TestParseSelector(t *testing.T) {
 			expected: specs.Platform{
 				OS:           "linux",
 				Architecture: defaultArch,
+				Variant:      defaultVariant,
 			},
-			formatted: joinNotEmpty("linux", defaultArch),
+			formatted: joinNotEmpty("linux", defaultArch, defaultVariant),
 		},
 		{
 			input: "s390x",
@@ -279,8 +285,9 @@ func TestParseSelector(t *testing.T) {
 			expected: specs.Platform{
 				OS:           "darwin",
 				Architecture: defaultArch,
+				Variant:      defaultVariant,
 			},
-			formatted: joinNotEmpty("darwin", defaultArch),
+			formatted: joinNotEmpty("darwin", defaultArch, defaultVariant),
 		},
 	} {
 		t.Run(testcase.input, func(t *testing.T) {