[CI] - test_integration_single.rb - fix curl msg regex

[MSP-Greg]

Description

Fix regex used to test curl output message when connection refused.

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[dentarg]

Motivation for this change: #3011 (comment)

[MSP-Greg]

Since CI logs are flushed based on age, below is the error that started appearing in CI on macOS 11, added some line breaks:

TestIntegrationSingle#test_term_not_accepts_new_connections
[/Users/runner/work/puma/puma/test/test_integration_single.rb:115]:
Expected /Connection refused/ to match
"  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
  0     0    0     0    0     0      0      0 --:--:-- --:--:-- --:--:--     0\r  0     0    0     0    0     0      0      0 --:--:-- --:--:-- --:--:--     0
curl: (7) Failed to connect to 127.0.0.1 port 51683 after 3 ms: Couldn't connect to server\n".

So rather than 'Connection refused', it's logging 'Couldn't connect to server'.