browser: Emit exit event in more of the tests

[mstoykov]

While looking at errors for #4114 I noticed that in a lot of the cases there will be a bunch of goroutines blocked on wating for exit

goroutine 22166 [chan receive, 9 minutes]:
go.k6.io/k6/js/modules/k6/browser/browser.(*browserRegistry).handleExitEvent(0xc00bdfdd40, 0xc001b00600?, 0xc004aa98f0?)
	D:/a/k6/k6/js/modules/k6/browser/browser/registry.go:333 +0x45
created by go.k6.io/k6/js/modules/k6/browser/browser.newBrowserRegistry in goroutine 2172
	D:/a/k6/k6/js/modules/k6/browser/browser/registry.go:250 +0x2b2

goroutine 30009 [chan receive, 7 minutes]:
go.k6.io/k6/js/modules/k6/browser/browser.(*browserRegistry).handleIterEvents(0xc00b8bb140, {0x23f02c8, 0x32048c0}, 0xc008c74930, 0xc00f89de60)
	D:/a/k6/k6/js/modules/k6/browser/browser/registry.go:264 +0x85
created by go.k6.io/k6/js/modules/k6/browser/browser.newBrowserRegistry in goroutine 5913
	D:/a/k6/k6/js/modules/k6/browser/browser/registry.go:251 +0x336

goroutine 47313 [select]:
go.k6.io/k6/js/modules/k6/browser/common.(*BaseEventEmitter).syncAll(0xc01267e400, {0x23f03e0, 0xc00990f3b0})
	D:/a/k6/k6/js/modules/k6/browser/common/event_emitter.go:126 +0xa5
created by go.k6.io/k6/js/modules/k6/browser/common.NewBaseEventEmitter in goroutine 47297
	D:/a/k6/k6/js/modules/k6/browser/common/event_emitter.go:116 +0x195

goroutine 21485 [chan receive, 9 minutes]:
go.k6.io/k6/js/modules/k6/browser/browser.(*browserRegistry).handleIterEvents(0xc0053fefc0, {0x23f02c8, 0x32048c0}, 0xc003526070, 0xc002b70d80)
	D:/a/k6/k6/js/modules/k6/browser/browser/registry.go:264 +0x85
created by go.k6.io/k6/js/modules/k6/browser/browser.newBrowserRegistry in goroutine 2175
	D:/a/k6/k6/js/modules/k6/browser/browser/registry.go:251 +0x336

goroutine 22857 [chan receive, 9 minutes]:
go.k6.io/k6/js/modules/k6/browser/browser.(*browserRegistry).handleIterEvents(0xc007796d20, {0x23f02c8, 0x32048c0}, 0xc0077d00e0, 0xc0077cc300)
	D:/a/k6/k6/js/modules/k6/browser/browser/registry.go:264 +0x85
created by go.k6.io/k6/js/modules/k6/browser/browser.newBrowserRegistry in goroutine 2169
	D:/a/k6/k6/js/modules/k6/browser/browser/registry.go:251 +0x336

Which likely is as the exit event is only emitted from the k6 cmd and that isn't emulated in the tests currently.

This might not be that big of a problem most times, but it definitely doesn't help with the stability of the tests or the ability to figure what happens.