os: define TestIssue60181 only on Unix platforms

os: define TestIssue60181 only on Unix platforms

In CL 517755 the test was added in the unconstrained os_test.go
because it appeared to be portable, but it turned out not to be
valid on plan9.

(The build error was masked on the misc-compile TryBots by #61923.)

Although the test can also compile and run on Windows, the bug it
checks for is specific to Linux and only really needs to run there, so
I am moving it to os_unix_test.go instead of adding yet another test
file for “Unix and Windows but not Plan 9”.

Updates #60181.

Change-Id: I41fd11b288217e95652b5daa72460c0d26bde606
Reviewed-on: https://go-review.googlesource.com/c/go/+/518255
Auto-Submit: Bryan Mills <bcmills@google.com>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>