Preventing tests using s3_impostor from running in parallel

[mmaslankaprv]

Preventing tests using s3_impostor from running in parallel with each other to prevent listen port conflict.

Backports Required

• [ ] none - not a bug fix
• [ ] none - this is a backport
• [x] none - issue does not exist in previous branches
• [ ] none - papercut/not impactful enough to backport
• [ ] v24.2.x
• [ ] v24.1.x
• [ ] v23.3.x

Release Notes

• none

[rockwotj]

ld.lld: error: duplicate symbol: unit_test_httpd_port_number()

Because s3_imposter already declares this you get errors. We could either:

• Make this a template arg to the httpd fixture
• Make the symbol weak so the compiler doesn't complain about it