Change parser interface to allow stat

Change parser interface to allow stat

Currently, the GRUB2 parser incorrectly reports "[ -f <path> ]" as
false if the size of the file is above 1 MB.  This patch changes the
parser interface to allow stating files (with parser_stat_file).  Then
in the implementation of "[ -f <path> ]", we can use parser_stat_file
instead of parser_request_file which has the size limitation.  I
eliminate parser_check_dir in lieu of this new interface, which has
the side effect of making "[ -d <path> ]" work (the error code for
stat was not checked correctly before).

I add a basic test for the test file operations -f, -s, and -d (to
show that my changes to test file operations do not break them) and
minorly modify the test framework to ensure it has enough fidelity to
cause the expected results.  Unfortunately the test wouldn't have
caught the issue with -d, since the test framework stubs out the
parser interface itself.  Nor can the test framework catch the initial
problem with -f because the imposed limit is (transitively) in
function parser_request_file.

Note that -f and -d follow symlinks despite the fact that GRUB does
not (see
http://lists.gnu.org/archive/html/grub-devel/2016-02/msg00142.html
discussing GRUB's behavior).  This is not a change to Petitboot's
behavior though.

Tested:
 The test test-grub2-test-file-ops passes.  I booted Petitboot against
 a GRUB snippet:

status=success

if [ ! -f /large_file -a $status = success ]
then status=fail_large_file
fi
if [ ! -d /a_directory -a $status = success ]
then status=fail_dir
fi

menuentry $status {
  linux /vmlinux
}

 (after making /large_file a file of size > 1 MiB and /a_directory a
 directory) and the menuentry had title "success", as desired.

Signed-off-by: Alan Dunn <amdunn@google.com>
Signed-off-by: Sam Mendoza-Jonas <sam@mendozajonas.com>