Skip `headers.forEach` tests for native Firefox implementation
Skip `headers.forEach` tests for native Firefox implementation

Mysteriously, it doesn't implement it.

Driftt/fetch,benestudio/fetch,snadn/fetch,zebulonj/fetch-plus,keyanzhang/fetch-xhr,Driftt/fetch,keyanzhang/fetch-xhr,snadn/fetch,Driftt/fetch,snadn/fetch,benestudio/fetch,JhansiYarnagula/dummy2,keyanzhang/fetch-xhr,snadn/fetch,JhansiYarnagula/dummy2,zebulonj/fetch-plus,aleclarson/fetch,github/fetch,zebulonj/fetch-plus,JhansiYarnagula/dummy2,benestudio/fetch