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

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Co-authored-by: Fabian Meumertzheim <fabian@meumertzhe.im>
2 files changed