tests: TestFetchNoStoreCacheControl fails on SemaphoreCI

[lucab]

Lately, test runs on SemaphoreCI have been reliably failing on TestFetchNoStoreCacheControl. Failure log in all cases is:

=== RUN   TestFetchNoStoreCacheControl
--- FAIL: TestFetchNoStoreCacheControl (2.11s)

panic: aci test server: failed to listen on localhost:443: [
(listen on 127.0.0.1:443, attempt #1: listen tcp 127.0.0.1:443: bind: address already in use)
(listen on ::1:443, attempt #1: listen tcp: too many colons in address ::1:443)
(listen on 127.0.0.1:443, attempt #2: listen tcp 127.0.0.1:443: bind: address already in use)
(listen on ::1:443, attempt #2: listen tcp: too many colons in address ::1:443)
] [recovered]

[lucab]

Bumping up attempts and pauses on retries doesn't mitigate this. TestDeferredSignatureDownload is affected by the same as it also uses fixed-port aci-server.