Fix non-idiomatic boolean comparisons

## Problem
Code uses `== true` and `== false` comparisons instead of idiomatic Go style.

## Locations
- `pkg/shell/shell.go:86` - `if beginFound == false`
- `pkg/junitxml/writer_test.go:24` - `if isSet == true`

## Fix
Use idiomatic Go boolean expressions:

```go
// Before
if beginFound == false {
if isSet == true {

// After
if !beginFound {
if isSet {
```

## Priority
Low - Code style