Add test suite endpoint to GraphQL

What does this MR do?

Adds ability to fetch a specific test suite from a pipeline via GraphQL. You can access it with following query:

query getPipelineTestSuite($fullPath: ID!, $pipelineIid: ID!, $buildIds: [ID!]!) {
  project(fullPath: $fullPath) {
    pipeline(iid: $pipelineIid) {
      testSuite(buildIds: $buildIds) {
        name
        totalTime
        totalCount
        successCount
        failedCount
        skippedCount
        errorCount
        suiteError
        testCases {
          nodes {
            status
            name
            classname
            file
            attachmentUrl
            executionTime
            stackTrace
            systemOutput
            recentFailures {
              count
              baseBranch
            }
          }
        }
      }
    }
  }
}

Screenshots (strongly suggested)

Screen_Cast_2021-04-08_at_9.57.19_AM

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
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• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
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• 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

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• 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