test: add platform name validation and version component count checks

Extend cross-platform tests to verify Platform.name returns one of seven known
OS strings and assert Process.version splits into exactly three numeric
components, while fixing missing trailing newline in isPosix test file.

test/os/process/version.wren

@@ -5,3 +5,4 @@ System.print(Process.version.isEmpty) // expect: false
 
 var isNumber = Fn.new {|str| Num.fromString(str) != null }
 System.print(Process.version.split(".").all(isNumber)) // expect: true
+System.print(Process.version.split(".").count) // expect: 3