[dev.typeparams] test: fix excluded files lookup so it works on Windows
[dev.typeparams] test: fix excluded files lookup so it works on Windows

Updates #43866.

Change-Id: I15360de11a48c6f23f25c5ff3a15c117a34127ff
Reviewed-on: https://go-review.googlesource.com/c/go/+/286034
Trust: Robert Griesemer <a6752097faebf1f02d80107d77a9be009089afe3@golang.org>
Run-TryBot: Robert Griesemer <a6752097faebf1f02d80107d77a9be009089afe3@golang.org>
Reviewed-by: Matthew Dempsky <e56ac2c500064424afac424d4b5e53bd0570a731@google.com>

golang/go,cloudflare/go,CAFxX/go,golang/go,golang/go,nawawi/go,golang/go,cloudflare/go,cloudflare/go,cloudflare/go,nawawi/go,cloudflare/go,cloudflare/go,CAFxX/go,golang/go,nawawi/go,nawawi/go,CAFxX/go,CAFxX/go,golang/go,golang/go,golang/go,CAFxX/go,cloudflare/go,cloudflare/go,nawawi/go,CAFxX/go,CAFxX/go,nawawi/go,nawawi/go,CAFxX/go,nawawi/go