unix: fix MmapPtr test failing on OpenBSD

[ncruces]

OpenBSD apparently doesn't allow unmapping address space if part of the region is already unmapped.
This tweaks the test so that munmapping twice no longer happens.

Fixes golang/go#68181

Cq-Include-Trybots: luci.golang.try:go1.22-openbsd-amd64

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Patch Set 1: Commit-Queue+1

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Patch Set 1:

Failed to compute tryjob requirement. Reason:

builder "golang/try/gotip-openbsd-amd64" is included but not defined in the LUCI project

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Patch Set 1: LUCI-TryBot-Result-1

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Patch Set 2: Commit message was updated.

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Patch Set 2: Commit-Queue+1

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Patch Set 2:

Dry run: CV is trying the patch.

Bot data: {"action":"start","triggered_at":"2024-06-26T13:53:53Z","revision":"1a6d22bc65215dc6e68b218c5f4806a486e51f7f"}

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This CL has passed the run

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Patch Set 3: Auto-Submit+1 Code-Review+2 Commit-Queue+1

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Patch Set 3:

Failed to compute tryjob requirement. Reason:

builder "golang/try/gotip-openbsd-amd64" is included but not defined in the LUCI project

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Patch Set 4: Auto-Submit+1 TryBot-Bypass+1

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This PR is being closed because golang.org/cl/595095 has been merged.