chore: Fixed lint issues in internal/testhelper/testhelper.go

This MR fixes the following lint issues in internal/testhelper/testhelper.go

Related Issue: #704

internal/testhelper/testhelper.go:31:29: Error return value of `os.Chdir` is not checked (errcheck)
        t.Cleanup(func() { os.Chdir(oldWd) })
                                   ^
internal/testhelper/testhelper.go:14:1: exported: exported function TempEnv should have comment or be unexported (revive)
func TempEnv(t *testing.T, env map[string]string) {
^
internal/testhelper/testhelper.go:20:1: exported: exported function PrepareTestRootDir should have comment or be unexported (revive)
func PrepareTestRootDir(t *testing.T) string {
^
internal/testhelper/testhelper.go:1:1: package-comments: should have a package comment (revive)
package testhelper
^
internal/testhelper/testhelper.go:52:14: ST1005: error strings should not be capitalized (stylecheck)
                return "", fmt.Errorf("Could not get caller info")
                           ^
shekhar@Shekhars-MacBook-Pro gitlab-shell % ./support/bin/golangci-lint-1.56.2 run internal/testhelper/testhelper.go
internal/testhelper/testhelper.go:1:1: package-comments: should have a package comment (revive)
package testhelper
^
internal/testhelper/testhelper.go:14:1: exported: exported function TempEnv should have comment or be unexported (revive)
func TempEnv(t *testing.T, env map[string]string) {
^
internal/testhelper/testhelper.go:20:1: exported: comment on exported function PrepareTestRootDir should be of the form "PrepareTestRootDir ..." (revive)
// Prepares the root directory for testing
^
internal/testhelper/testhelper.go:56:14: ST1005: error strings should not be capitalized (stylecheck)
                return "", fmt.Errorf("Could not get caller info")
                           ^