Refactor to k6 lookupEnv and abstract env. var. lookup

[inancgumus]

This PR refactors the direct usage of os.LookupEnv to k6 LookupEnv. Please have a look at the commit descriptions for the changes needed.

These new changes provide enhanced testability and consistency throughout the module.

The following are all the environment variables and options that the k6-browser module supports. The outdated XK6_* are removed and aligned with the new browser options we have.

// Execution specific.
const (
	// InstanceScenarios is an environment variable that can be used to
	// define the extra scenarios details to use when running remotely.
	InstanceScenarios = "K6_INSTANCE_SCENARIOS"

	// WebSocketURLs is an environment variable that can be used to
	// define the WS URLs to connect to when running remotely.
	WebSocketURLs = "K6_BROWSER_WS_URL"

	// BrowserArguments is an environment variable that can be used to
	// pass extra arguments to the browser process.
	BrowserArguments = "K6_BROWSER_ARGS"

	// BrowserExecutablePath is an environment variable that can be used
	// to define the path to the browser to execute.
	BrowserExecutablePath = "K6_BROWSER_EXECUTABLE_PATH"

	// BrowserEnableDebugging is an environment variable that can be used to
	// define if the browser should be launched with debugging enabled.
	BrowserEnableDebugging = "K6_BROWSER_DEBUG"

	// BrowserHeadless is an environment variable that can be used to
	// define if the browser should be launched in headless mode.
	BrowserHeadless = "K6_BROWSER_HEADLESS"

	// BrowserIgnoreDefaultArgs is an environment variable that can be
	// used to define if the browser should ignore default arguments.
	BrowserIgnoreDefaultArgs = "K6_BROWSER_IGNORE_DEFAULT_ARGS"

	// BrowserGlobalTimeout is an environment variable that can be used
	// to set the global timeout for the browser.
	BrowserGlobalTimeout = "K6_BROWSER_TIMEOUT"
)

// Logging and debugging.
const (
	// EnableProfiling is an environment variable that can be used to
	// enable profiling for the browser. It will start up a debugging
	// server on ProfilingServerAddr.
	EnableProfiling = "K6_BROWSER_ENABLE_PPROF"

	// ProfilingServerAddr is the address of the profiling server.
	ProfilingServerAddr = "localhost:6060"

	// LogCaller is an environment variable that can be used to enable
	// the caller function information in the browser logs.
	LogCaller = "K6_BROWSER_LOG_CALLER"

	// LogLevel is an environment variable that can be used to set the
	// log level for the browser logs.
	LogLevel = "K6_BROWSER_LOG"

	// LogCategoryFilter is an environment variable that can be used to
	// filter the browser logs based on their category. It supports
	// regular expressions.
	LogCategoryFilter = "K6_BROWSER_LOG_CATEGORY_FILTER"
)

[ankur22]

LGTM 🙂

[ka3de]

Great work @inancgumus ! 👏 Made a couple of comments, but overall LGTM.