Add more parser test cases

[JLHwung]

Q	A
License	MIT

This PR aims for improving @babel/parser's test coverage. Ideally the only uncovered lines should be throw new Error("unreachable").

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[JLHwung]

These checks are redundant because shouldParseExportDeclaration will check isAsyncFunction

babel/packages/babel-parser/src/parser/statement.js

Lines 1918 to 1938 in a827ca4

	shouldParseExportDeclaration(): boolean {
	if (this.match(tt.at)) {
	this.expectOnePlugin(["decorators", "decorators-legacy"]);
	if (this.hasPlugin("decorators")) {
	if (this.getPluginOption("decorators", "decoratorsBeforeExport")) {
	this.unexpected(this.state.start, Errors.DecoratorBeforeExport);
	} else {
	return true;
	}
	}
	}
	return (
	this.state.type.keyword === "var" ||
	this.state.type.keyword === "const" ||
	this.state.type.keyword === "function" ||
	this.state.type.keyword === "class" ||
	this.isLet() ||
	this.isAsyncFunction()
	);
	}

When this.isContextual("async") is true, isAsyncFunction() must be true and async must be followed by a function token, therefore this.unexpected(next, tt._function); is never reached.