Add build reference to test reports

What does this MR do?

Because requirements reports will be processed per each CI build, not per pipeline, it will be better to associate each test report with a build.

In a next step we can also remove pipeline reference to normalize this table.

This column will be used in !33031 (merged) Related to #215513 (comment 348942649)

DB migration

$ rake db:migrate
== 20200527135313 AddRequirementsBuildReference: migrating ====================
-- add_column(:requirements_management_test_reports, :build_id, :bigint)
   -> 0.0008s
-- add_index(:requirements_management_test_reports, :build_id, {:name=>"index_requirements_management_test_reports_on_build_id"})
   -> 0.0071s
-- add_foreign_key(:requirements_management_test_reports, :ci_builds, {:column=>:build_id, :on_delete=>:nullify})
   -> 0.0091s
== 20200527135313 AddRequirementsBuildReference: migrated (0.0201s) ===========

$ rake db:rollback
== 20200527135313 AddRequirementsBuildReference: reverting ====================
-- remove_column(:requirements_management_test_reports, :build_id)
   -> 0.0008s
== 20200527135313 AddRequirementsBuildReference: reverted (0.0027s) ===========

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team