`setLenUninit(string)` was broken on the legacy refc backend when
growing within existing spare capacity.
`setLengthStrUninit` in `lib/system/sysstr.nim` only updated len when it
had to reallocate or when shrinking.
If oldLen < newLen <= capacity, it returned early without finalizing:
```nim
var s = newStringOfCap(10)
s.add("abc")
s.setLenUninit(6)
doAssert s.len == 6 # used to fail, len stayed 3
```
This escaped `tests/stdlib/tstring.nim` because the testing routine
`checkSetLenUninit` mostly resizes strings created at **exact**
length/capacity, so growth usually took the reallocating branch.
The new regression test covers the missing edge case.
So sorry for catching this only on the day of the stable release! In my
defense, the original PR hung in limbo for quite a while and it didn't
spend enough time in devel after the merge.
cbe8ce59ed