For those from 2020+ it now can be achieved as simple as that:
{{ join "," .Values.some.array }}
It produces the correct output:
value: "test1,test2,test3"
At least it works with (more or less) recent versions of helm-cli:
Client: &version.Version{SemVer:"v2.16.2", GitCommit:"bbdfe5e7803a12bbdf97e94cd847859890cf4050", GitTreeState:"clean"}