Reach 100 percent coverage of unit test

[uier]

• model/auth.py Test/auth #214
• model/test.py #215
• Analyze the service usage and remove unused scripts and APIs #217
• test: announcement #218
• test health #219
• Test contest #221
• Test: homework #223
• remove: unused /contest API #224
• Test copycat #226
• test model/user #227
• Test course #230
• Remove BS_Detect #231
• Remove inbox #234
• refactor: remove create_users.py #236
• Test post #237
• Test problem #239
• test model/utils/smtp.py #242
• test model/utils/request.py #243
• test model/profile.py #245
• test mongo/user.py #247
• test mongo/utils.py #252
• test mongo/sandbox.py #257
• test submission API / model #259
• test mongo/base.py #261
• test mongo/ip_filter.py #265
• test mongo/engine.py #267
• ...feel free to add new tasks here for reaching 100%