Fix filesystem test on macOS (#460)

Author: neonichu

Sources/TSCBasic/FileSystem.swift

@@ -103,6 +103,8 @@ public extension FileSystemError {
             self.init(.noEntry, path)
         case TSCLibc.ENOTDIR:
             self.init(.notDirectory, path)
+        case TSCLibc.EEXIST:
+            self.init(.alreadyExistsAtDestination, path)
         default:
             self.init(.ioError(code: errno), path)
         }

Tests/TSCBasicTests/FileSystemTests.swift

@@ -331,7 +331,13 @@ class FileSystemTests: XCTestCase {
                 _ = try fs.readFileContents(root)
 
             }
-            XCTAssertThrows(FileSystemError(.isDirectory, root)) {
+            #if os(macOS)
+            // Newer versions of macOS end up with `EEXISTS` instead of `EISDIR` here.
+            let expectedError = FileSystemError(.alreadyExistsAtDestination, root)
+            #else
+            let expectedError = FileSystemError(.isDirectory, root)
+            #endif
+            XCTAssertThrows(expectedError) {
                 try fs.writeFileContents(root, bytes: [])
             }
             XCTAssert(fs.exists(filePath))