Golden test pretty-printing sometimes generated invalid `HashSet` constructor

[lihaoyi]

Seems to happen with large sets, which then fail to compile because HashSet is not imported by default. Impor

[854/870] libs.tabcomplete.test.compile
[854] [info] compiling 1 Scala source to /Users/lihaoyi/Github/mill/out/libs/tabcomplete/test/compile.dest/classes ...
[854] [error] -- [E006] Not Found Error: /Users/lihaoyi/Github/mill/libs/tabcomplete/test/src/mill/tabcomplete/TabCompleteTests.scala:171:10 
[854] [error] 171 |          HashSet(
[854] [error]     |          ^^^^^^^
[854] [error]     |          Not found: HashSet
[854] [error]     |
[854] [error]     | longer explanation available when compiling with `-explain`
[854] [error] -- [E006] Not Found Error: /Users/lihaoyi/Github/mill/libs/tabcomplete/test/src/mill/tabcomplete/TabCompleteTests.scala:206:10 
[854] [error] 206 |          HashSet(
[854] [error]     |          ^^^^^^^
[854] [error]     |          Not found: HashSet
[854] [error]     |
[854] [error]     | longer explanation available when compiling with `-explain`
[854] [error] two errors found

Importing it manually via import scala.collection.immutable.HashSet works, but we should probably just special case this to generate Set(...)