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

(cherry picked from commit a0dfb02)
(cherry picked from commit de5642b63e5c9210052b97105457a3debf5cc309)
(cherry picked from commit c6e94cb7910d5f12bd64c21cd4ed36499c6addf1)
(cherry picked from commit 359fed1)

Author: bluca

test/units/testsuite-71.sh

@@ -31,7 +31,7 @@ testcase_hostname() {
     if [[ -n "$orig" ]]; then
         assert_in "Static hostname: $orig" "$(hostnamectl)"
     fi
-    assert_in "Kernel: $(uname -s) $(uname -r)" "$(hostnamectl)"
+    assert_in "Kernel: $(uname -s) $(uname -r | sed 's/\+/\\+/g')" "$(hostnamectl)"
 
     # change hostname
     assert_rc 0 hostnamectl set-hostname testhost