bzltestutil: document init() behavior change in Go 1.21 (#3655) * bzltestutil: document init() behavior change in Go 1.21 Also adjust test_chdir accordingly. * Update go/tools/bzltestutil/init.go Co-authored-by: Fabian Meumertzheim <fabian@meumertzhe.im> * fixup doc * adjust doc --------- Co-authored-by: Fabian Meumertzheim <fabian@meumertzhe.im>