tests/libtest-core: support multiple literal checks

`grep` supports checking multiple fixed strings separated by newlines,
but it's mostly just easier to pass them as separate arguments, so let's
support that. This is now at parity with the similar
`assert_file_has_content`.

Closes: #1409
Approved by: cgwalters

tests/libtest-core.sh

@@ -112,9 +112,12 @@ assert_file_has_content () {
 }
 
 assert_file_has_content_literal () {
-    if ! grep -q -F -e "$2" "$1"; then
-        _fatal_print_file "$1" "File '$1' doesn't match fixed string list '$2'"
-    fi
+    fpath=$1; shift
+    for s in "$@"; do
+        if ! grep -q -F -e "$s" "$fpath"; then
+            _fatal_print_file "$fpath" "File '$fpath' doesn't match fixed string list '$s'"
+        fi
+    done
 }
 
 assert_file_has_mode () {