Fix/linter errs test browser

[inancgumus]

This improvement PR fixes the linter errors in the test browser.

It also starts using t.Helper and t.Fatalf instead of panicking in the test. Benefits: Easily pinpoint the problematic code line in the tests and test browser with a leaner fatal message.

Example:

kbtf -run='TestPageSetExtraHTTPHeaders'
--- FAIL: TestPageSetExtraHTTPHeaders (0.30s)
    page_test.go:96: You should enable HTTP test server, see: withHTTPServer option

Instead of:

kbtf -run='TestPageSetExtraHTTPHeaders'
--- FAIL: TestPageSetExtraHTTPHeaders (0.32s)
panic: You should enable HTTP test server, see: withHTTPServer option [recovered]
	panic: You should enable HTTP test server, see: withHTTPServer option

goroutine 7 [running]:
testing.tRunner.func1.2({0x105189f00, 0x105333f10})
	/usr/local/go/src/testing/testing.go:1209 +0x258
testing.tRunner.func1(0x140005091e0)
	/usr/local/go/src/testing/testing.go:1212 +0x284
panic({0x105189f00, 0x105333f10})
	/usr/local/go/src/runtime/panic.go:1038 +0x21c

# .... hundreds of lines more

Related: grafana/k6#4496

[imiric]

👍