We should always verify error codes returned by function calls in our
test suite to not accidentally miss any weird results. Coverity reported
missing checks in several locations, which this commit fixes.

Our file utils functions all have a "futils" prefix, e.g.
`git_futils_touch`. One would thus naturally guess that their
definitions and implementation would live in files "futils.h" and
"futils.c", respectively, but in fact they live in "fileops.h".

Rename the files to match expectations.

Opening a default config when ~/.gitconfig doesn't exist, locking it,
and attempting to write to it causes an assertion failure.

Treat non-existent global config file content as an empty string.

According to reports, libgit2 is unable to read a global configuration
file that is simply a symlink to the real configuration. Write a
(succeeding) test that shows that libgit2 _is_ correctly able to do so.

While we do verify that we are able to open the global ".gitconfig" file
in config::global::open_global, we never verify that we it is in fact
readable. Do so by writing the global configuration file and verifying
that reading from it produces the expected values.

The config::global test suite creates various different directories and
files which are being populated with pretend-global files.
Unfortunately, the tests do not clean up after themselves, which may
cause subsequent tests to fail due to cruft left behind.

Fix this by always removing created directories and their contents.

Query the `GIT_CONFIG_LEVEL_PROGRAMDATA` location when setting it up
for tests, in case the test runner has sandboxed it.

This is where portable git stores the global configuration which we can
use to adhere to it even though git isn't quite installed on the system.