Fix usage strings for #5692

[rwdj]

Went through the list in #5692 and found inconsistencies for unlink, runcon, test, and hashsum.

runcon uutils:

runcon [CONTEXT COMMAND [ARG...]]

but runcon requires a context command to function:

runcon: No command is specified

unlink uutils:

unlink [FILE]

but unlink requires a file to unlink:

error: the following required arguments were not provided:
  <FILE>

hashsum:

hashsum [OPTIONS] [FILE]...

but hashsum requires a digest to operate and may take multiple options:

hashsum: Needs an algorithm to hash with.

test uutils:

test EXPRESSION
[
[ EXPRESSION ]
[ ]
[ OPTION
]

but:

1. [ alone is not a valid test command:

    [: missing ']'
   

2. test alone is a valid test command.

3. ] alone is not a valid test command.

As appropriate, this matches GNU behavior.

[rwdj]

Just noticed test in that list. Trying to figure out how to get it to spit out the usage message.

[rwdj]

Added test usage string fixes.

[sylvestre]

[ alone is not a valid test command:

I think it is legit :)

test can be symlink to [
which is used in bash like in

if [ foo ]; then

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/misc/stdbuf (fails in this run but passes in the 'main' branch)

[rwdj]

@sylvestre While true, I think you misunderstood me. I'm not saying [, but only [.

Neither Bash nor uutil will accept only [ as a command. They both result in:

[: missing `]'

I'm willing to bet uutil's behavior here was actually modeled after Bash than GNU.

GNU test, from what I can figure, will accept only [, although it isn't documented. But I'll double check that in a bit. Either way, uutils's documented usage is different from GNU's test documented usage.

So, (assuming I'm correct about GNU test behavior which I'm going to double check) there's one of two options: match with GNU's undocumented acceptance of just [, or update uutil to match GNU's documentation and uutil's behavior.

[rwdj]

I figured out how to disable the [ builtin in bash:

enable -n [

And then it can be checked with:

$ [ --version

uutils, Bash, and GNU [ all report:

$ [
[: missing ‘]’

So, it's only uutil's documentation that's inconsistent