`rune` is alias for `int32`. `randString()` used `rand.Int()`
which returns system-specific non-negative integer number. For
64-bit systems it will be int64. An attempt to cast `int64` to
`int32` (`rune`) leads the latter to be a negative number in
case of overflow. This caused the resulting string containing
unexpected symbols.
Signed-off-by: Ekaterina Lebedeva <ekaterina.lebedeva@yadro.com>
