fix(temp): use isolatedModules in ts-jest

`isolatedModules` being set to true will fix a memory leak in ts-jest. However, type checking will not be performed whilst running tests (will still have type checking when writing tests).

Revert after the memory leak is fixed.
See kulshekhar/ts-jest#1967
See: https://kulshekhar.github.io/ts-jest/user/config/isolatedModules

jest.config.js

@@ -12,4 +12,11 @@ module.exports = {
       statements: 38, // Increase this percentage as test coverage improves
     },
   },
+  globals: {
+    // Revert when memory leak in ts-jest is fixed.
+    // See https://github.com/kulshekhar/ts-jest/issues/1967.
+    'ts-jest': {
+      isolatedModules: true,
+    },
+  },
 }