[lld-macho][test] Make reloc-subtractor.s robust

The test requires that LLVM integreated assembler generates
SUBTRACTOR/UNSIGNED relocations for `.long _minuend_1 - _subtrahend_1`.
This currently works by luck because:

* `_minuend_1` and `_subtrahend_1` are in different fragments (separated by a MCFillFragment)
* and the result is known to be negative at parsing time.

D153096 will change the assembler to fold `.long _minuend_1 - _subtrahend_1` to
a constant, giving ld -order_file no chance to change the result.

To fix the test, move the referenced labels after the label differences to block
constant folding.

Note: you may think the model is somewhat broken and it is. The
.subsections_via_symbols mechanism does not block such constant folding. In
reality SUBTRACTOR/UNSIGNED is for references to another section, which does not
have the problem.

Reviewed By: #lld-macho, smeenai

Differential Revision: https://reviews.llvm.org/D153382