Fix false positive -Wrestrict warning in test_ja4.cc with GCC 12 (#12690)

Assign 'a' directly instead of "a" to avoid the false positive
-Wrestrict warning in GCC 12.

Author: bryancall

plugins/experimental/ja4_fingerprint/test_ja4.cc

@@ -230,7 +230,7 @@ TEST_CASE("JA4")
           "when we create a JA4 fingerprint, "
           "then indices [8,9] thereof should contain \"aa\".")
   {
-    TLS_summary.ALPN = "a";
+    TLS_summary.ALPN = 'a';
     CHECK("aa" == call_JA4(TLS_summary).substr(8, 2));
   }