Tests: adapting js_fetch_objects.t for slow machines.

The test is too slow to trigger timeout in perl tests
when the following condition are met:
    * ASAN build, with ASAN_OPTIONS=fast_unwind_on_malloc=0
    * -DNJS_DEBUG_MEMORY -DNGX_DEBUG_PALLOC -DNGX_DEBUG_MALLOC in CFLAGS

Previously, invalid header name test produced many JS exception.
While looking for symbol names NJS allocates/deallocates a lot of memory.
In combination with the conditions above this might result in a timeout.

The fix is to simplify invalid header name test.

nginx/t/js_fetch_objects.t

@@ -139,30 +139,8 @@ $t->write_file('test.js', <<EOF);
                 return h.get('a');
              }, 'X'],
             ['invalid header name', () => {
-                const valid = "!#\$\%&'*+-.^_`|~0123456789";
-
-                for (var i = 0; i < 128; i++) {
-                    var c = String.fromCodePoint(i);
-
-                    if (valid.indexOf(c) != -1 || /[a-zA-Z]+/.test(c)) {
-                        continue;
-                    }
-
-                    try {
-                        new Headers([[c, 'a']]);
-                        throw new Error(
-                                   `header with "\${c}" (\${i}) should throw`);
-
-                    } catch (e) {
-                        if (e.message != 'invalid header name') {
-                            throw e;
-                        }
-                    }
-                }
-
-                return 'OK';
-
-             }, 'OK'],
+                var h = new Headers([[':', 'a']]);
+             }, 'invalid header name'],
             ['invalid header value', () => {
                 var h = new Headers({A: 'aa\x00a'});
              }, 'invalid header value'],