Correct Headers.prototype[Symbol.iterator] test

[annevk]

Per feedback in #4523 the original test would never be able to pass.

[wpt-pr-bot]

Notifying @jdm and @youennf. (Learn how reviewing works.)

[wpt-stability-bot]

Firefox

Testing revision 1d212a7bef7e823551932fb6e53accec562dfe45
Starting 10 test iterations
All results were stable

All results

/fetch/api/headers/headers-basic.html

Subtest	Results
	OK
Create headers with existing headers with custom iterator	FAIL
Create headers with existing headers	PASS
Check get method	PASS
Create headers from no parameter	PASS
Create headers from empty object	PASS
Create headers with record	PASS
Check forEach method	FAIL
Check has method	PASS
Check values method	FAIL
Check append method	PASS
Create headers with 1 should throw	PASS
Check set method	PASS
Check Symbol.iterator method	FAIL
Check entries method	FAIL
Create headers from undefined parameter	PASS
Check delete method	PASS
Create headers with null should throw	PASS
Check keys method	FAIL
Create headers with sequence	PASS

[wpt-stability-bot]

Chrome

Testing revision 1d212a7bef7e823551932fb6e53accec562dfe45
Starting 10 test iterations
All results were stable

All results

/fetch/api/headers/headers-basic.html

Subtest	Results
	OK
Create headers with existing headers with custom iterator	FAIL
Create headers with existing headers	PASS
Check get method	PASS
Create headers from no parameter	PASS
Create headers from empty object	PASS
Create headers with record	PASS
Check forEach method	PASS
Check has method	PASS
Check values method	FAIL
Check append method	PASS
Create headers with 1 should throw	PASS
Check set method	PASS
Check Symbol.iterator method	PASS
Check entries method	FAIL
Create headers from undefined parameter	FAIL
Check delete method	PASS
Create headers with null should throw	PASS
Check keys method	FAIL
Create headers with sequence	PASS