bug#23537: timeout test gets false-positive for duration of -1.189731495357231765e+4932

[Jim Meyering]

On systems with recent glibc, this abuse of timeout elicits the expected error:

  $ src/timeout -- -1.189731495357231765e+4932 sleep 0
  src/timeout: invalid time interval ‘-1.189731495357231765e+4932’
  Try 'src/timeout --help' for more information.

But with glibc-2.12's strtod, that input maps to a double-precision
value of 0 rather than to -inf, so timeout does this:

  $ src/timeout -- -1.189731495357231765e+4932 sleep 0; echo $?
  0

Similarly, the sleep.sh test fails because even without the leading "-",
that number ($LDBL_MAX) maps to 0:

  $ src/timeout 0.1 sleep 1.189731495357231765e+4932; echo $?
  0

which causes two tests to fail:

  tests/misc/timeout-parameters
  tests/misc/sleep

I see a couple of ways to avoid trouble.
Perhaps the most general is to make gnulib's strtod module detect and
compensate for these errors.
But that's CentOS6-era glibc, so maybe not worth it for such a corner case.