tests: fix exec fails when grep exists with status other than 0 (#9066)

Co-authored-by: lizhaolong.lzl <lizhaolong.lzl@B-54MPMD6R-0221.local>

tests/unit/moduleapi/fork.tcl

@@ -1,7 +1,11 @@
 set testmodule [file normalize tests/modules/fork.so]
 
 proc count_log_message {pattern} {
-    set result [exec grep -c $pattern < [srv 0 stdout]]
+    set status [catch {exec grep -c $pattern < [srv 0 stdout]} result]
+    if {$status == 1} {
+        set result 0
+    }
+    return $result
 }
 
 start_server {tags {"modules"}} {