improve tests just a bit (#79)

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