Workaround an issue regarding libXCTest.so

We're seeing an issue where the XCTest library doesn't use absolute paths to link dependent Swift libs on Linux which means we need to explicitly set `LD_LIBRARY_PATH` to work around that.

Author: neonichu

Sources/Commands/Utilities/TestingSupport.swift

@@ -133,6 +133,13 @@ enum TestingSupport {
         if let location = toolchain.xctestPath {
             env.prependPath("Path", value: location.pathString)
         }
+        #elseif os(Linux)
+        var libraryPaths = ["/usr/lib/swift/linux"]
+        if let path = env["PATH"], let firstPathEntry = path.components(separatedBy: ":").first {
+            libraryPaths.append("\(firstPathEntry)/../lib/swift/linux")
+        }
+        // Pass this explicitly on Linux because XCTest started requiring it, rdar://103054033
+        env["LD_LIBRARY_PATH"] = libraryPaths.joined(separator: ":")
         #endif
         return env
         #else