os: os_test: test fails when /bin/pwd does not exist

[rminnich]

What version of Go are you using (go version)?

Any version from the last few years, currently 1.9

Does this issue reproduce with the latest release?

yes

What operating system and processor architecture are you using (go env)?

amd64
various distros that no longer have /bin

What did you do?

go test os

Try it on, e.g., arch, or u-root (u-root.tk) where pwd is not in /bin

What did you expect to see?

go test os passes

What did you see instead?

It fails as /bin/pwd does not exist

[gopherbot]

Change https://golang.org/cl/60010 mentions this issue: os: don't assume /bin/pwd in test, find it in $PATH