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/platform/is_posix.wren

@@ -2,4 +2,4 @@ import "os" for Platform
 
 // Can't test for certain values since this test is cross-platform, but we can
 // at least make sure it is callable and returns a bool.
-System.print(Platform.isPosix is Bool) // expect: true
+System.print(Platform.isPosix is Bool) // expect: true

test/os/platform/name.wren

@@ -1,5 +1,16 @@
 import "os" for Platform
 
+var platforms = [
+    "Windows",
+    "iOS",
+    "OS X",
+    "Unknown",
+    "Linux",
+    "Unix",
+    "POSIX"
+]
+
 // Can't test for certain values since this test is cross-platform, but we can
 // at least make sure it is callable and returns a string.
 System.print(Platform.name is String) // expect: true
+System.print(platforms.contains(Platform.name)) // expect: true