chore: enhance jest test reliability (#800)

* feat: manually set new mongo db server per test setup

instead of relying on @shelf/mongodb

* 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

* ref: use mongodb-memory-server-core instead of mongodb-memory-server

* chore(tests): inline mongo memory server generation

* chore: remove unused jest-environment-node dependency

.gitignore

@@ -56,10 +56,6 @@ tmp/
 # =======
 .localstack
 
-# @shelf/jest-mongodb generated file
-# =======
-globalConfig.json
-
 # Tests
 # =======
 coverage/

jest.config.js

@@ -1,14 +1,9 @@
-const { defaults: tsJestPreset } = require('ts-jest/presets')
-
 module.exports = {
-  preset: '@shelf/jest-mongodb',
+  preset: 'ts-jest',
   testMatch: ['**/?(*.)+(spec|test).[t]s?(x)'],
   modulePaths: ['<rootDir>'],
+  testEnvironment: 'node',
   testPathIgnorePatterns: ['<rootDir>/dist/', '<rootDir>/node_modules/'],
-  transform: {
-    // Needed to use @shelf/jest-mongodb preset.
-    ...tsJestPreset.transform,
-  },
   collectCoverageFrom: ['./src/**/*.{ts,js}', '!**/__tests__/**'],
   coveragePathIgnorePatterns: ['./node_modules/', './tests'],
   coverageReporters: ['lcov', 'text'],
@@ -17,5 +12,11 @@ module.exports = {
       statements: 38, // Increase this percentage as test coverage improves
     },
   },
-  testEnvironment: 'node',
+  globals: {
+    // Revert when memory leak in ts-jest is fixed.
+    // See https://github.com/kulshekhar/ts-jest/issues/1967.
+    'ts-jest': {
+      isolatedModules: true,
+    },
+  },
 }