[ftr] provider a wrapper for supertest with specific role and headers

[dmlemeshko]

@azasypkin suggested a nice improvement in test design: getSupertestWithRoleScope('admin', ...options...) that would do all the heavy lifting with custom, internal and security headers internally.

    describe('against an application page', () => {
      it(`uses compression when there isn't a referer`, async () => {
        const response = await getSupertestWithRoleScope('admin', {withInternalHeaders: true})
          .get('/app/kibana')
          .set('accept-encoding', 'gzip')
        expect(response.header).to.have.property('content-encoding', 'gzip');
      });
    });

[elasticmachine]

Pinging @elastic/appex-qa (Team:QA)