test: improve automated test coverage

[svietry]

Overall coverage: 67.8% -> 73.4% (+5.6 percentage points)

New test files created

1. internal/config/user_config_test.go - 10 test cases
   - TestParseUserConf - token, private_key, unknown, empty string inputs
   - TestUserConf_String - valid enums and out-of-range values
2. internal/config/kube_config_test.go - 24 test cases
   - TestKubeConf_IsKubeConfigFromFile - path set/empty
   - TestKubeConf_IsNonFileKubeConfigValid - 7 field combination scenarios
   - TestKubeConfUser_isValid - PrivateKey, ServiceAccountToken, default
   - TestKubeConf_GetKubeConfigFromConf_* - ServiceAccountToken, PrivateKey, invalid base64
   - TestKubeConf_getAuthInfosFromConf_* - invalid base64 client cert and private key
   - TestKubeConfUser_MarshalJSON/YAML_RedactsSensitiveFields - redaction verification

Existing test files extended

3. pkg/lib_test.go - 9 new test cases
   - TestReportToStdout - stdout JSON encoding verification
   - TestHandleReport - 4 scenarios: empty account, invalid config, successful post, account route override
   - TestHandleReport_VerboseOutput - verbose flag triggers stdout output
   - TestSetLogger - global logger assignment
4. internal/anchore/anchoreclient_test.go - 3 new test cases
   - TestAPIClientError_Error - basic, with API details, with controller details