add some unit tests for yurt tunnel

[YRXING]

What type of PR is this?

/kind bug
/kind good-first-issue
/kind feature

What this PR does / why we need it:

improve the reliability of yurt-tunnel.

Which issue(s) this PR fixes:

Fixes #416

Special notes for your reviewer:

Does this PR introduce a user-facing change?

NONE

[openyurt-bot]

Welcome @YRXING! It looks like this is your first PR to openyurtio/openyurt 🎉

[rambohe-ch]

@YRXING Please add the license header for new files. btw: fix the lint errors and unit-test failed cases.

[rambohe-ch]

@YRXING please merge these six commits into one commit.

[rambohe-ch]

@YRXING please fix the following errors.

[rambohe-ch]

@YRXING please merge all commits into one commit.

[YRXING]

@YRXING please fix the following errors.

I have format the project but golangci-lint still failed.

[rambohe-ch]

@YRXING please fix the following errors.

I have format the project but golangci-lint still failed.

@YRXING On my local desktop, the golangci-lint errors as following:

helinbodeMacBook-Pro:openyurt helinbo$ golangci-lint run --out-format=github-actions --disable-all  -E deadcode -E gofmt -E goimports -E ineffassign -E misspell -E vet --timeout=15m
::error file=pkg/yurttunnel/dns/util.go,line=116::File is not `gofmt`-ed with `-s` (gofmt)
::error file=pkg/profile/profile_test.go,line=20::File is not `goimports`-ed (goimports)
::error file=pkg/yurttunnel/handlerwrapper/tracerequest/tracereq_test.go,line=20::File is not `goimports`-ed (goimports)
::error file=pkg/yurttunnel/dns/util_test.go,line=20::File is not `goimports`-ed (goimports)

[rambohe-ch]

@YRXING please fix the following errors.

I have format the project but golangci-lint still failed.

@YRXING you need to install goimports command and reformat the above code files. and if you use goland to edit golang code, you can right click above files and select Go Tools --> Goimports File to format files.

[rambohe-ch]

/lgtm
/approve

[openyurt-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: rambohe-ch, YRXING

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[rambohe-ch]

@YRXING Would you like to apply OpenYurt community member, and you can add me as sponsor. apply example is here: openyurtio/community#24