testhelper: Add new `Unsetenv()` helper

With Go 1.17 a new `t.Setenv()` helper was introduced that automatically
restores the previous environment variable and that verifies that the
current test is not labelled as parallel. We're about to migrate all
callers to use it instead of `testhelper.ModifyEnvironment()`.

One part that `testhelper.ModifyEnvironment()` does though is to unset
an environment variable in case the given value is the empty string. And
unfortunately, Go didn't introduce a `t.Unsetenv()` helper at the same
time.

Implement a new function `testhelper.Unsetenv()` that behaves the same
as `t.Setenv()`, except that it unsets the environment variable.

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