我的代码现已修复，并且运行良好：

我的新File类：

'use strict'

const fs = require('fs')

module.exports = class File {
    constructor() {}

    async savePicture(filename,imageData) {
        if (filename === undefined || filename === '') throw new Error(`filename can't be empty`)
        if (imageData === undefined || imageData === '') throw new Error(`imageData can't be empty`)
        try {
            fs.writeFileSync(filename,imageData,'binary')
        } catch(err) {
            throw err
        }
    }

    async readPicture(filename) {
        if (filename === undefined || filename === '') throw new Error(`filename can't be empty`)
        try {
            return fs.readFileSync(filename,'binary')
        } catch(err) {
            if (err.message === `ENOENT: no such file or directory,open '${filename}'`)
                throw new Error("file doesn't exist")
            else
                throw err
        }
    }
}

我的新单元测试文件：

'use strict'

const mock = require('mock-fs')
const File = require('../modules/file')

const fs = require('fs')
const file = new File()


beforeAll( async() => {
    mock({
        'test': {
            'foobar': 'foobar2\n'
        }
    })
})

afterAll( async() => {
    mock.restore()
})

describe('savePicture()',() => {
    beforeEach( async() => {})
    afterEach( async() => {})

    test(`filename can't be empty string`,async done => {
        expect.assertions(1)
        try {
            await file.savePicture('','asd')
            done.fail('test failed')
        } catch(err) {
            expect(err.message).toBe(`filename can't be empty`)
        } finally {
            done()
        }
    })
    test(`filename can't be undefined`,async done => {
        expect.assertions(1)
        try {
            await file.savePicture()
            done.fail('test failed')
        } catch(err) {
            expect(err.message).toBe(`filename can't be empty`)
        } finally {
            done()
        }
    })
    test(`imageData can't be empty`,async done => {
        expect.assertions(1)
        try {
            await file.savePicture('foo.jpg')
            done.fail('test failed')
        } catch(err) {
            expect(err.message).toBe(`imageData can't be empty`)
        } finally {
            done()
        }
    })
    test(`binary information has to be correctly saved`,async done => {
        expect.assertions(1)
        try {
            await file.savePicture('image','raboof')
            let output = await file.readPicture('image')
            expect(output).toBe('raboof')
        } catch(err) {
            expect(err.message).toBe("issue with writing")
        } finally {
            done()
        }
    })
})

describe('readPicture()',() => {
    beforeEach( async() => { })
    afterEach( async() => { })
    test(`filename can't be empty`,async done => {
        expect.assertions(1)
        try {
            await file.readPicture('')
            done.fail('test failed')
        } catch(err) {
            expect(err.message).toBe(`filename can't be empty`)
        } finally {
            done()
        }
    })
    test(`filename can't be undefined`,async done => {
        expect.assertions(1)
        try {
            await file.readPicture()
            done.fail('test failed')
        } catch(err) {
            expect(err.message).toBe(`filename can't be empty`)
        } finally {
            done()
        }
    })
    test('file has to exist',async done => {
        expect.assertions(1)
        try {
            await file.readPicture('nonExistent.txt')
            done.fail('test failed')
        } catch(err) {
            expect(err.message).toBe("file doesn't exist")
        } finally {
            done()
        }
    })
    test(`binary information has to be correctly read`,async done => {
        expect.assertions(1)
        try {
            let output = await file.readPicture('test/foobar')
            expect(output).toBe("foobar2\n")
        } catch(err) {
            console.log(err)
            expect(err.message).toBe("issue with reading")
        } finally {
            done()
        }
    })
})