[ISSUE-84] support helper

[siller174]

1. Support Helper() in StepCtx
2. Add helper to Error() and ErrorF
   This is needed for correct tracing inside the test.

Test: bla_test.go

func TestName(t *testing.T) {
	runner.Run(t, "Single test with allure-go Runner", func(t provider.T) {
		t.Epic("Compare with allure-go")
		t.Description("New Test Description")
		t.WithNewStep("Step description", func(ctx provider.StepCtx) {
			ctx.Errorf("Error message")
		})
	})
}

func TestName1(t *testing.T) {
	runner.Run(t, "Single test with allure-go Runner", func(t provider.T) {
		t.Errorf("Error message")
	})
}

Logs without helper:

=== RUN   TestName
=== RUN   TestName/Single_test_with_allure-go_Runner
    common.go:125: Error message
--- FAIL: TestName (0.00s)
    --- FAIL: TestName/Single_test_with_allure-go_Runner (0.00s)


=== RUN   TestName1
=== RUN   TestName1/Single_test_with_allure-go_Runner
    common.go:125: Error message
--- FAIL: TestName1 (0.00s)
    --- FAIL: TestName1/Single_test_with_allure-go_Runner (0.00s)

Trace from allure: common.go:125: Error message and common.go:125: Error message

Log with helper:

=== RUN   TestName
=== RUN   TestName/Single_test_with_allure-go_Runner
    bla_test.go:15: Error message
--- FAIL: TestName (0.00s)
    --- FAIL: TestName/Single_test_with_allure-go_Runner (0.00s)


=== RUN   TestName1
=== RUN   TestName1/Single_test_with_allure-go_Runner
    bla_test.go:22: Error message 
--- FAIL: TestName1 (0.00s)
    --- FAIL: TestName1/Single_test_with_allure-go_Runner (0.00s)

Trace from test: bla_test.go:15: Error message and bla_test.go:22: Error message