TestBuildAddTarXz: xz with '-k' due to msys bug

TestBuildAddTarXz/TestBuildAddTarXzGz make exec calls like
'cd %SOME_TEMP_DIR%; xz test.gz' but this fails when called
from integration-cli tests (doesn't fail from msys or cmd shell).

It fails with error: "xz: test.tar: Cannot remove: Permission denied."
Proposing workaround of using the `xz --keep` argument since keeping
test.tar is harmless and it's already in os.TempDir().

Signed-off-by: Ahmet Alp Balkan <ahmetalpbalkan@gmail.com>

integration-cli/docker_cli_build_test.go

@@ -3877,7 +3877,7 @@ func TestBuildAddTarXz(t *testing.T) {
 		if err := tw.Close(); err != nil {
 			t.Fatalf("failed to close tar archive: %v", err)
 		}
-		xzCompressCmd := exec.Command("xz", "test.tar")
+		xzCompressCmd := exec.Command("xz", "-k", "test.tar")
 		xzCompressCmd.Dir = tmpDir
 		out, _, err := runCommandWithOutput(xzCompressCmd)
 		if err != nil {
@@ -3930,7 +3930,7 @@ func TestBuildAddTarXzGz(t *testing.T) {
 			t.Fatalf("failed to close tar archive: %v", err)
 		}
 
-		xzCompressCmd := exec.Command("xz", "test.tar")
+		xzCompressCmd := exec.Command("xz", "-k", "test.tar")
 		xzCompressCmd.Dir = tmpDir
 		out, _, err := runCommandWithOutput(xzCompressCmd)
 		if err != nil {