allow x-pack plugins to import core service mocks (#36771)

using relative imports from x-pack to kibana core doesn't work atm.
although we cannot use absolute imports, because jest cannot match
them internally.

x-pack/dev-tools/jest/create_jest_config.js

@@ -22,6 +22,7 @@ export function createJestConfig({
     ],
     moduleNameMapper: {
       '^ui/(.*)': `${kibanaDirectory}/src/legacy/ui/public/$1`,
+      '^src/core/(.*)': `${kibanaDirectory}/src/core/$1`,
       '^plugins/xpack_main/(.*);': `${xPackKibanaDirectory}/plugins/xpack_main/public/$1`,
       '\\.(jpg|jpeg|png|gif|eot|otf|webp|svg|ttf|woff|woff2|mp4|webm|wav|mp3|m4a|aac|oga)$':
         `${kibanaDirectory}/src/dev/jest/mocks/file_mock.js`,