Add negative-zero.c test to document ksh bug

[laurenthuberdeau]

Context

While working on #64, I tried running the compile the tests using pnut.sh (instead of pnut-sh compiled by gcc as it is usually done) and found that pnut.sh would compile the tests/_all/preprocessor/if/if.c test differently. Code guarded by a #elif block was not included because the flag we used to check if a previous condition was true was -0 and the code compared it to 0 using string comparison (as an optimization) which are obviously different.

This PR adds a test so we can track this bug.

This simple script also shows the bug:

a=0
: $(( b = -a ))
: $(( c = +a ))
echo $a # print 0
echo $b # print -0 instead of 0
echo $a # print 0 as expected

I have only been able to reproduce the issue when using ksh on ARM. Other shells and ksh in a x86 VM show the expected behavior. This is similar to this issue found by @leo-ard where an undefined behavior in ksh produced different results in the x86 and ARM versions, and it may be indirectly caused by the use of floating point numbers to do all arithmetic in ksh.

Ksh version:

$ ksh --version
  version         sh (AT&T Research) 93u+ 2012-08-01