test/nginx: extract request() and add testing

[alxndrsn]

Added as a precursor to #1949.

What has been done to verify that this works as intended?

This does not change functional code, just adds a test for request() and extracts that function to its own file.

These tests can be seen to fail by substituting fetch() for request():

--- a/test/nginx/src/mocha/request.spec.js
+++ b/test/nginx/src/mocha/request.spec.js
@@ -4,9 +4,10 @@ const {
   assert,
 } = require('../lib');
 
-const request = require('./request');
+//const request = require('./request');
+const request = fetch;

  request()
    1) should not follow redirects
    2) should allow setting Host header


  0 passing (53ms)
  2 failing

  1) request()
       should not follow redirects:
     TypeError: fetch failed
  
     Caused by: Error: getaddrinfo ENOTFOUND example.test
      at GetAddrInfoReqWrap.onlookupall [as oncomplete] (node:dns:122:26)

  2) request()
       should allow setting Host header:

      AssertionError: expected '127.0.0.1:36363' to equal 'not-a-host'
      + expected - actual

      -127.0.0.1:36363
      +not-a-host
      
      at Context.<anonymous> (nginx/src/mocha/request.spec.js:68:12)
      at process.processTicksAndRejections (node:internal/process/task_queues:104:5)

Why is this the best possible solution? Were any other approaches considered?

request() is quite complicated, and deserves tests before future changes.

How does this change affect users? Describe intentional changes to behavior and behavior that could have accidentally been affected by code changes. In other words, what are the regression risks?

No effect - just test code.

Does this change require updates to documentation? If so, please file an issue here and include the link below.

No.

Before submitting this PR, please make sure you have:

• branched off and targeted the next branch OR only changed documentation/infrastructure (master is stable and used in production)
• verified that any code or assets from external sources are properly credited in comments or that everything is internally sourced

[alxndrsn]

Reviewer: this is a bug in request(), but I didn't want to change any functionality in this PR.