Fix test_server spec helper to not leak fiber

[straight-shoota]

Extracted from #7197

Refactors test_server spec helper to avoid leaking a fiber.

As described in #7197 (comment) load testing this change introduces Connection reset by peer errors on MacOS (@asterite Which version exactly did you test it on?). That's why this was extracted from the previous PR in order to merge the other features while this needs more investigation.

I can't debug on MacOS, so I'd be happy to get some help here.

[asterite]

Mac OSX Mojave. I just ran it locally:

$ bin/crystal spec spec/std/http/client/client_spec.cr 
............E......

Failures:

  1) HTTP::Client doesn't read the body if request was HEAD

       Error reading socket: Connection reset by peer (Errno)
         from src/socket.cr:59:9 in 'unbuffered_read'
         from src/io/buffered.cr:201:5 in 'fill_buffer'
         from src/io/buffered.cr:90:7 in 'peek'
         from src/io.cr:632:37 in 'gets'
         from src/io.cr:591:5 in 'gets'
         from src/http/client/response.cr:25:11 in 'from_io?'
         from src/http/client.cr:562:5 in 'exec_internal_single'
         from src/http/client.cr:548:5 in 'exec_internal'
         from src/http/client.cr:544:5 in 'exec'
         from src/http/client.cr:666:5 in 'exec'
         from src/http/client.cr:370:3 in 'get'
         from spec/std/http/client/client_spec.cr:184:20 in '->'
         from src/spec/methods.cr:255:3 in 'it'
         from spec/std/http/client/client_spec.cr:179:5 in '->'
         from src/spec/context.cr:255:3 in 'describe'
         from src/spec/methods.cr:16:5 in 'describe'
         from spec/std/http/client/client_spec.cr:46:3 in '__crystal_main'
         from src/crystal/main.cr:97:5 in 'main_user_code'
         from src/crystal/main.cr:86:7 in 'main'
         from src/crystal/main.cr:106:3 in 'main'
       

Finished in 582.55 milliseconds
19 examples, 0 failures, 1 errors, 0 pending

Failed examples:

crystal spec spec/std/http/client/client_spec.cr:179 # HTTP::Client doesn't read the body if request was HEAD