test: count assert_output_contains matches as fixed strings

Replace the bash pattern-substitution loop with grep -F -o piped to wc -l so the helper counts literal substring occurrences instead of treating the expected value as a glob pattern. The old implementation interpreted `[`, `]`, `*`, `?`, and `\` as pattern syntax, which made `assert_output_contains "['task.py', 'test']"` report 17 matches against an output that contained the string exactly once - the inner characters were being matched as a character class. assert_output_not_contains delegates to assert_output_contains and is fixed transitively.

tests/unit/test_helper.bash

@@ -143,11 +143,8 @@ assert_output_contains() {
   local input="$output"
   local expected="$1"
   local count="${2:-1}"
-  local found=0
-  until [ "${input/$expected/}" = "$input" ]; do
-    input="${input/$expected/}"
-    found=$((found + 1))
-  done
+  local found
+  found=$(printf '%s' "$input" | grep -F -o -- "$expected" | wc -l | tr -d ' ')
 
   if [[ "$count" -eq -1 ]]; then
     if [[ "$found" -gt 0 ]]; then

tests/unit/traefik.bats

@@ -468,7 +468,7 @@ teardown() {
   assert_success
   assert_output_contains "dns provider cf_api_email"
   assert_output_contains "dns provider cf_api_key"
-  assert_output_contains "*******"
+  assert_output_contains "*******" 2
   assert_output_not_contains "test@example.com"
   assert_output_not_contains "secret-key"