1. In vector test, prepare RTZ answer for RTZ rounding.
2. In mixed_format_random test, for a given float 'arg', the test
previously used 'arg' directly to generate ref_str:
```
ref_str << str_sprintf(format, arg);
```
This approach incorrectly assumes:
```
(float) arg == to_fp(to_str(arg));
```
However, this assumption fails under RTZ rounding. For example:
```
arg = 0xC642549C
to_str(arg) = -12437.152343f
to_fp_rtz(-12437.152343f) = 0xC642549B (-0X1.84A936P+13)
to_fp_rte(-12437.152343f) = 0xC642549C (-0X1.84A938P+13)
```
To address this, the reference result is now computed based on the
literal float string rather than the original 'arg' value.
54afc2e7a5