OZG-6592: Introduce main-tests as app-variation in admin. Generalize run-tests.sh.

alfa-client/Jenkinsfile.e2e

alfa-client/apps/admin-e2e/cypress.config.json

...pps/admin-e2e/src/e2e/main-tests/app/0-login-logout.cy.ts

...ent/apps/admin-e2e/src/e2e/main-tests/app/buildinfo.cy.ts

.../src/e2e/main-tests/benutzer_rollen/benutzer_rollen.cy.ts

...apps/admin-e2e/src/e2e/main-tests/postfach/signatur.cy.ts

alfa-client/apps/alfa-e2e/cypress.config.json

alfa-client/apps/run-tests.sh