Add tests around Linux distribution detection.

* Add a series of tests for checking distribution logic.

Author: kcieplak

Sources/SWBGenericUnixPlatform/Plugin.swift

@@ -126,10 +126,18 @@ struct GenericUnixSDKRegistryExtension: SDKRegistryExtension {
                 defaultProperties = [:]
             }
 
-            if operatingSystem == .freebsd || (operatingSystem == .linux && operatingSystem.distribution?.kind == .amazon && operatingSystem.distribution?.version == "2") || operatingSystem != context.hostOperatingSystem {
-                // FreeBSD is always LLVM-based, use lld
-                // Amazon Linux 2 has a gold linker bug see: https://sourceware.org/bugzilla/show_bug.cgi?id=23016, use lld
-                // or if we're cross-compiling, use lld
+            let shouldUseLLD = {
+                switch operatingSystem {
+                case .freebsd:
+                    return true // FreeBSD is always LLVM-based.
+                case .linux where operatingSystem.distribution?.kind == .amazon && operatingSystem.distribution?.version == "2":
+                    return true // Amazon Linux 2 has a gold linker bug see: https://sourceware.org/bugzilla/show_bug.cgi?id=23016.
+                default:
+                    return operatingSystem != context.hostOperatingSystem // Cross-compiling
+                }
+            }()
+
+            if shouldUseLLD {
                 defaultProperties["ALTERNATE_LINKER"] = "lld"
             }