为什么有时我会得到“不是已知元素”警告？

Question

我有一项使用业力和茉莉花测试的服务。

title.service.spec

import { TestBed } from '@angular/core/testing';
import { UserPanelComponent } from '../layout/user-panel/user-panel.component';
import { MatExpansionModule } from '@angular/material/expansion';
import { TitleService } from './title.service';

describe('TitleService', () => {
  beforeEach(() => TestBed.configureTestingModule({
    declarations: [ UserPanelComponent ],
    imports: [ MatExpansionModule ],
  }));

  it('should be created', () => {
    const service: TitleService = TestBed.get(TitleService);
    expect(service).toBeTruthy();
  });
});

title.service.ts

import { Injectable } from '@angular/core';

@Injectable({
  providedIn: 'root'
})
export class TitleService {
 private Value;
    constructor() {
        this.Value = 'shiny beautiful string';
    }
    setValue(val) {
        this.Value = val;
    }
    getValue() {
        return this.Value ;
    }
}

i运行测试，一切都很好。输出：

  TitleService
    √ should be created

我进行了另一项测试，我得到了：

  TitleService
    √ should be created
WARN LOG: ''mat-expansion-panel' is not a known element:
1. If 'mat-expansion-panel' is an Angular component, then verify that it is part of this module.
2. If 'mat-expansion-panel' is a Web Component then add 'CUSTOM_ELEMENTS_SCHEMA' to the '@NgModule.schemas' of this component to suppress this message.'
WARN: ''mat-expansion-panel' is not a known element:
1. If 'mat-expansion-panel' is an Angular component, then verify that it is part of this module.
2. If 'mat-expansion-panel' is a Web Component then add 'CUSTOM_ELEMENTS_SCHEMA' to the '@NgModule.schemas' of this component to suppress this message.'
WARN LOG: ''mat-expansion-panel-header' is not a known element:
1. If 'mat-expansion-panel-header' is an Angular component, then verify that it is part of this module.
2. If 'mat-expansion-panel-header' is a Web Component then add 'CUSTOM_ELEMENTS_SCHEMA' to the '@NgModule.schemas' of this component to suppress this message.'
WARN: ''mat-expansion-panel-header' is not a known element:
1. If 'mat-expansion-panel-header' is an Angular component, then verify that it is part of this module.
2. If 'mat-expansion-panel-header' is a Web Component then add 'CUSTOM_ELEMENTS_SCHEMA' to the '@NgModule.schemas' of this component to suppress this message.'
WARN LOG: ''mat-panel-description' is not a known element:
1. If 'mat-panel-description' is an Angular component, then verify that it is part of this module.
2. If 'mat-panel-description' is a Web Component then add 'CUSTOM_ELEMENTS_SCHEMA' to the '@NgModule.schemas' of this component to suppress this message.'
WARN: ''mat-panel-description' is not a known element:
1. If 'mat-panel-description' is an Angular component, then verify that it is part of this module.
2. If 'mat-panel-description' is a Web Component then add 'CUSTOM_ELEMENTS_SCHEMA' to the '@NgModule.schemas' of this component to suppress this message.'

因此，总而言之，在测试时并非每次都会出现警告。为什么？如何永久摆脱它们？