Fix jest conf,

firebase/firebase-js-sdk#5687 (comment)

test/unit/jest.conf.js

@@ -16,8 +16,13 @@ module.exports = {
     '.*\\.(vue)$': '<rootDir>/node_modules/vue-jest'
   },
   testPathIgnorePatterns: [
-    '<rootDir>/test/e2e'
+    '<rootDir>/test/e2e',
+    '/node_modules/*',
+    '/node_modules',
+    '<rootDir>/node_modules/*',
+    '<rootDir>/node_modules/(?!(@firebase.*)/)'
   ],
+  resolver: '<rootDir>/test/unit/my-module-resolve.js',
   snapshotSerializers: ['<rootDir>/node_modules/jest-serializer-vue'],
   setupFiles: ['<rootDir>/test/unit/setup'],
   mapCoverage: true,

test/unit/my-module-resolve.js

@@ -0,0 +1,19 @@
+// my-module-resolve.js
+module.exports = (request, options) => {
+    // Call the defaultResolver, so we leverage its cache, error handling, etc.
+    return options.defaultResolver(request, {
+      ...options,
+      // Use packageFilter to process parsed `package.json` before the resolution (see https://www.npmjs.com/package/resolve#resolveid-opts-cb)
+      packageFilter: pkg => {
+        if (pkg.name.startsWith('@firebase')) {
+          return {
+            ...pkg,
+            // Alter the value of `main` before resolving the package
+            main: pkg.esm5 || pkg.module
+          }
+        }
+
+        return pkg
+      }
+    })
+  }