test: add more tests for pkg/app/middlewares/server/basic_auth

[mamil]

What type of PR is this?

test: Adding missing tests or correcting existing tests

Check the PR title.

• This PR title match the format: <type>(optional scope): <description>
• The description of this PR title is user-oriented and clear enough for others to understand.

(Optional) Translate the PR title into Chinese.

为pkg/app/middlewares/server/basic_auth添加更多单元测试

(Optional) More detail description for this PR(en: English/zh: Chinese).

en: Add BasicAuth related tests in pkg/app/middlewares/server/basic_auth, basic_auth.go reaches 100% coverage
zh(optional): 添加pkg/app/middlewares/server/basic_auth中BasicAuth相关的测试，basic_auth.go达到100%覆盖

Which issue(s) this PR fixes:

#257

[codecov]

Codecov Report

Base: 67.53% // Head: 67.63% // Increases project coverage by +0.10% 🎉

Coverage data is based on head (9f5b7df) compared to base (d601515).
Patch has no changes to coverable lines.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #405      +/-   ##
===========================================
+ Coverage    67.53%   67.63%   +0.10%     
===========================================
  Files           89       89              
  Lines         8608     8608              
===========================================
+ Hits          5813     5822       +9     
+ Misses        2435     2426       -9     
  Partials       360      360              

Impacted Files	Coverage Δ
pkg/common/compress/compress.go	80.95% <0.00%> (-2.86%)	⬇️
...kg/app/middlewares/server/basic_auth/basic_auth.go	100.00% <0.00%> (+54.54%)	⬆️

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[CLAassistant]

All committers have signed the CLA.

[welkeyever]

Please resign the CLA🙏

[mamil]

@welkeyever CLA signed

[welkeyever]

LGTM, thx~