all: ensure that tests do not write to the current directory #28387
Description
In #27957, @hyangah noticed that the tests for compress/bzip2 fail when GOROOT is not writable, and those tests are run whenever we run go test all in module mode (which is intended to be a useful default).
As noted in #28386, tests should not assume that they can write to the current directory. We should ensure that none of the tests in the standard library make that mistake.