cmd/link: reliably remove temporary directory in testDwarf
cmd/link: reliably remove temporary directory in testDwarf

We were using t.Parallel in a subtest, which meant that the main test
would not wait for the subtest, so the main test would delete the
temporary directory before the subtest used it. The subtest worked
because "go build -o /tmp/x/y/p.exe p" creates /tmp/x/y as needed.

Updates #30500

Change-Id: I5904ecac748d15ded4cb609f049fa548b8916a0e
Reviewed-on: https://go-review.googlesource.com/c/164857
Run-TryBot: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

akutz/go,josharian/go,akutz/go,golang/go,josharian/go,akutz/go,CAFxX/go,cloudflare/go,nawawi/go,josharian/go,CAFxX/go,josharian/go,akutz/go,golang/go,nawawi/go,josharian/go,golang/go,cloudflare/go,nawawi/go,akutz/go,cloudflare/go,golang/go,CAFxX/go,golang/go,CAFxX/go,CAFxX/go,CAFxX/go,cloudflare/go,golang/go,akutz/go,nawawi/go,akutz/go,CAFxX/go,josharian/go,golang/go,cloudflare/go,nawawi/go,CAFxX/go,cloudflare/go,josharian/go,golang/go,cloudflare/go,cloudflare/go,nawawi/go,nawawi/go,akutz/go,josharian/go,nawawi/go