tests: fix type promotion on 32-bit arches in http test code

[charles2910]

As reported in the bug:

On a 32-bit architecture (i386):

1. chunks * chunk_size evaluates to unsigned int (32-bit
   unsigned).
2. Due to Usual Arithmetic Conversions in C, the signed -1 (int)
   operand of the ternary operator is promoted to unsigned int,
   yielding 4294967295 (0xFFFFFFFF).
3. The ternary operator returns 4294967295 as an unsigned int.
4. This unsigned value is then assigned to r->clength (apr_off_t,
   64-bit signed). Since the source is unsigned, it is zero-extended,
   resulting in r->clength becoming +4294967295.
5. The subsequently executed check if(r->clength >= 0) evaluates to
   true.
6. Inside the block, apr_ltoa(r->pool, (long)r->clength) casts it to
   a 32-bit signed long (on 32-bit platforms), which truncates it
   back to -1, formatting it as "-1" and sending the
   "Content-Length: -1" header.

On a 64-bit architecture (amd64), size_t is 64-bit, and -1 (32-bit
int) is promoted to unsigned long (64-bit), yielding
18446744073709551615. When assigned to r->clength (64-bit signed),
it wraps back to -1, which correctly skips the Content-Length
generation.

This type promotion mismatch can be safely fixed by avoiding the
signed/unsigned mixture in the ternary operator.

Origin: debian, https://bugs.debian.org/1140793
Bug-Debian: https://bugs.debian.org/1140793

These are standard Debian patches' trailers, I can get rid of then if you prefer.

I've tested the fix using debusine on i386 runner:

782s tests/http/test_05_errors.py::TestErrors::test_05_04_unclean_tls_shutdown[http/1.0] PASSED [ 17%]
782s tests/http/test_05_errors.py::TestErrors::test_05_04_unclean_tls_shutdown[http/1.1] PASSED [ 18%]
782s tests/http/test_05_errors.py::TestErrors::test_05_04_unclean_tls_shutdown[h2] PASSED [ 18%]

Versus the i386 run in Debian's infrastructure:

2690s tests/http/test_05_errors.py::TestErrors::test_05_04_unclean_tls_shutdown[http/1.0] FAILED [ 17%]
2690s tests/http/test_05_errors.py::TestErrors::test_05_04_unclean_tls_shutdown[http/1.1] PASSED [ 18%]
2690s tests/http/test_05_errors.py::TestErrors::test_05_04_unclean_tls_shutdown[h2] PASSED [ 18%]

[vszakats]

Tested the type bump method (#22210 (comment)), but it's no good, chunk_size is better kept as size_t.

(Ref: #22359 (comment))

[vszakats]

Thanks Charles, merged now!