[lib/test.js] Support URLs without leading slash

[petermikitsh]

I hit a weird issue and a cryptic error message when using this library. Decided to fix to simplify its usage.

Test Code:

import app from '../../../app'; // a plain old express app
import request from 'supertest';

test('happy path: ', async () => {
  const agent = request.agent(app);
  const res = await agent
    .get('api/tenants/startauth');
});

Output:

  ● happy path: 

    connect ECONNREFUSED 0.0.0.0:80

       7 |   const agent = request.agent(app);
       8 |   const res = await agent
    >  9 |     .host('0.0.0.0')
         |                                         ^
      10 |     .get('api/tenants/startauth');
      11 |     

Adding a leading slash to my get call fixed the problem. But with additional logging, I noticed the serverAddress function returned the invalid URL http://127.0.0.1:57625api/tenants/startauth.

This PR allows for the leading slash to be optional, fixing the URL generation and makes the cryptic error go away -- a better experience for all users of this library.

[coveralls]

Pull Request Test Coverage Report for Build 334

• 2 of 2 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.01%) to 97.297%

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Totals
Change from base Build 330:	0.01%
Covered Lines:	137
Relevant Lines:	141

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💛 - Coveralls

[rimiti]

@gjohnson could you check it please? 🙏