test(e2e): fix DevServer test

[ylemkimon]

• This is a bugfix
• This is a feature
• This is a code refactor
• This is a test update
• This is a docs update
• This is a metadata update

For Bugs and Features; did you add new tests?

N/A

Motivation / Use-Case

testBin sets the config path if the second argument is not provided:

webpack-dev-server/test/helpers/test-bin.js

Lines 15 to 20 in 1dde662

	function testBin(testArgs, configPath) {
	const cwd = process.cwd();
	const env = process.env.NODE_ENV;
	if (!configPath) {
	configPath = basicConfigPath;

If the config path is set via the first argument (testArgs), it'll result in multiple config paths, leading to flaky tests and invalid results, e.g.:

  ● DevServer › should supports entry as descriptor

    expect(received).toContain(expected) // indexOf

    Expected substring: "foo.js"
    Received string:    ""

      52 |       .then((output) => {
      53 |         expect(output.exitCode).toEqual(0);
    > 54 |         expect(output.stdout).toContain('foo.js');
         |                               ^
      55 |         done();
      56 |       })
      57 |       .catch(done);

      at test/e2e/DevServer.test.js:54:31
          at runMicrotasks (<anonymous>)

Breaking Changes

N/A

Additional Info

[codecov]

Codecov Report

Merging #3096 (e4e4680) into master (1dde662) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3096   +/-   ##
=======================================
  Coverage   92.30%   92.30%           
=======================================
  Files          37       37           
  Lines        1261     1261           
  Branches      328      328           
=======================================
  Hits         1164     1164           
  Misses         92       92           
  Partials        5        5           

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[snitin315]

Thanks!