os: TestProgWideChdir failing on Plan 9

[0intro]

Running TestProgWideChdir (CL 6382) on Plan 9 leads to a failure
of TestAppend and TestSameFile.

TestAppend and TestSameFile are failing because they are accessing
a file from a relative path, but the current working directory has been
deleted at the end of TestProgWideChdir.

% go test -v -run 'Test(ProgWideChdir|Append|SameFile)'
=== RUN TestProgWideChdir
Chdir /tmp/test401904176
 → getwd /tmp/test401904176
Chdir /usr/go/src/os
  → getwd /usr/go/src/os
Remove /tmp/test401904176
 → getwd /usr/go/src/os
 → Getwd /usr/go/src/os
--- PASS: TestProgWideChdir (0.03s)
=== RUN TestAppend
fixwdLocked
 → getwd err file has been removed
fixwdLocked
 → getwd err file has been removed
Remove append.txt
 → getwd
 → Getwd /usr/go/src/os
--- FAIL: TestAppend (0.01s)
    os_test.go:1330: Open: open append.txt: 'append.txt' file has been removed
=== RUN TestSameFile
fixwdLocked getwd err file has been removed
--- FAIL: TestSameFile (0.00s)
    os_test.go:1416: Create(a): open a: 'a' file has been removed
FAIL
exit status: 'os.test 631609: 1'
FAIL    os  0.095s

At the end of TestProgWideChdir, Chdir is calling chdir to change
the working directory for the current process and set wdStr.
Then the directory is removed.

In TestAppend, it calls Remove, which should fix the working directory.
However, fixwdLocked calls getwd which returns an error, because
the working directory was not yet changed in this process and this
directory was removed.

https://github.com/golang/go/blob/bc9748ee6b/src/syscall/pwd_plan9.go#L28

Since getwd is returning an error, fixwdLocked returns and never calls
chdir(wdStr) to fix the working directory.

I think we should ignore errors from getwd in fixwdLocked to always
be able to fix the working directory.