Close the file in fileio.exp test

I see the following fail in fileio.exp on mingw32 host gdb,

rename 1: ret = -1, errno = 13^M
^M
Breakpoint 2, stop () at fileio.c:76^M
76      static void stop () {}^M
(gdb) FAIL: gdb.base/fileio.exp: Rename a file

the test fails to rename a file which is not expected.  The previous
test test_write doesn't close the file, so the rename fails as a
result on Windows.  This patch fixes it by closing file in test_write,
and the fail goes away.

rename 1: ret = 0, errno = 0 OK^M
^M
Breakpoint 2, stop () at fileio.c:76^M
76      static void stop () {}^M
(gdb) PASS: gdb.base/fileio.exp: Rename a file

gdb/testsuite:

2014-10-29  Yao Qi  <yao@codesourcery.com>

	* gdb.base/fileio.c (test_write): Close the file.