test: escape '+' from uname -r before using in regex via '=~'

test: escape '+' from uname -r before using in regex via '=~'

It will otherwise interpret characters such as '+' as if they
were regex, and break matching.

For example TEST-71-HOSTNAME started failing when
uname -r started including a '+':

FAIL: 'Kernel: Linux 6.12.33+deb13-amd64' not found in:
 Static hostname: host
       Icon name: computer-vm
         Chassis: vm 🖴
      Machine ID: e6471c1f95ab49098827498ed816cdf7
         Boot ID: 231da987f64d4af59f2076fbb638c611
  Virtualization: kvm
Operating System: Debian GNU/Linux 13 (trixie)
          Kernel: Linux 6.12.33+deb13-amd64
    Architecture: x86-64