tests: cli: properly check for exit status codes in error cases

POSIX explicitly requires "set -e" to NOT treat "! cmd" as an error even
if fails[1]:

> The -e setting shall be ignored when executing the compound list
> following the while, until, if, or elif reserved word, *a pipeline
> beginning with the ! reserved word*, or any command of an AND-OR list
> other than the last. *[emphasis added]*

And bash has similar documentation on this behaviour[2].

This means that our tests were completely ineffective at detecting error
codes from gomtree, and as a result we did not detect the regression in
commit 83c9fdb78b ("refactor: prefactor for adding new subcommands").

The simplest solution (as done in this patch) is to just wrap all of the
failing examples in a subshell, which causes the shell to no longer
consider them exempt from "set -e". A more complete solution would be to
either switch to something like bats entirely or at least use something
like their "run" helper function to test for exit status codes
correctly.

[1]: https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#set
[2]: https://www.gnu.org/software/bash/manual/html_node/The-Set-Builtin.html

Fixes: 5d7f6c36e0 ("walk: directory is expected to be walked. A file is not.")
Fixes: 2d841d54bf ("test: testing the double -f comparison")
Fixes: f6c295f2e9 ("test: cli: add unicode verification test")
Fixes: 071977cef6 ("test: cli: add xattr tests")
Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>

test/cli/0011.sh

@@ -16,7 +16,7 @@ touch ${t}/foo
 
 ## can not walk a file. We're expecting a directory.
 ## https://github.com/vbatts/go-mtree/issues/166
-! ${gomtree} -c -K uname,uid,gname,gid,type,nlink,link,mode,flags,xattr,xattrs,size,time,sha256 -p ${t}/foo
+(! ${gomtree} -c -K uname,uid,gname,gid,type,nlink,link,mode,flags,xattr,xattrs,size,time,sha256 -p ${t}/foo)
 
 popd
 rm -rf ${t}