Remove FIXME comments to use in-memory file system

I don’t see any issue with using the real file system because it’s closer to what will be done in the real world and it makes it easier to debug issues because you can run `swift build` on the temporary directory.

Author: ahoppen

Tests/SKSwiftPMWorkspaceTests/SwiftPMWorkspaceTests.swift

@@ -106,7 +106,6 @@ final class SwiftPMWorkspaceTests: XCTestCase {
   }
 
   func testBasicSwiftArgs() async throws {
-    // FIXME: should be possible to use InMemoryFileSystem.
     let fs = localFileSystem
     try await withTestScratchDir { tempDir in
       try fs.createFiles(
@@ -166,7 +165,6 @@ final class SwiftPMWorkspaceTests: XCTestCase {
   }
 
   func testBuildSetup() async throws {
-    // FIXME: should be possible to use InMemoryFileSystem.
     let fs = localFileSystem
     try await withTestScratchDir { tempDir in
       try fs.createFiles(
@@ -212,7 +210,6 @@ final class SwiftPMWorkspaceTests: XCTestCase {
   }
 
   func testManifestArgs() async throws {
-    // FIXME: should be possible to use InMemoryFileSystem.
     let fs = localFileSystem
     try await withTestScratchDir { tempDir in
       try fs.createFiles(
@@ -245,7 +242,6 @@ final class SwiftPMWorkspaceTests: XCTestCase {
   }
 
   func testMultiFileSwift() async throws {
-    // FIXME: should be possible to use InMemoryFileSystem.
     let fs = localFileSystem
     try await withTestScratchDir { tempDir in
       try fs.createFiles(
@@ -283,7 +279,6 @@ final class SwiftPMWorkspaceTests: XCTestCase {
   }
 
   func testMultiTargetSwift() async throws {
-    // FIXME: should be possible to use InMemoryFileSystem.
     let fs = localFileSystem
     try await withTestScratchDir { tempDir in
       try fs.createFiles(
@@ -349,7 +344,6 @@ final class SwiftPMWorkspaceTests: XCTestCase {
   }
 
   func testUnknownFile() async throws {
-    // FIXME: should be possible to use InMemoryFileSystem.
     let fs = localFileSystem
     try await withTestScratchDir { tempDir in
       try fs.createFiles(
@@ -385,7 +379,6 @@ final class SwiftPMWorkspaceTests: XCTestCase {
   }
 
   func testBasicCXXArgs() async throws {
-    // FIXME: should be possible to use InMemoryFileSystem.
     let fs = localFileSystem
     try await withTestScratchDir { tempDir in
       try fs.createFiles(
@@ -479,7 +472,6 @@ final class SwiftPMWorkspaceTests: XCTestCase {
   }
 
   func testDeploymentTargetSwift() async throws {
-    // FIXME: should be possible to use InMemoryFileSystem.
     let fs = localFileSystem
     try await withTestScratchDir { tempDir in
       try fs.createFiles(
@@ -522,7 +514,6 @@ final class SwiftPMWorkspaceTests: XCTestCase {
   }
 
   func testSymlinkInWorkspaceSwift() async throws {
-    // FIXME: should be possible to use InMemoryFileSystem.
     let fs = localFileSystem
     try await withTestScratchDir { tempDir in
       try fs.createFiles(
@@ -577,7 +568,6 @@ final class SwiftPMWorkspaceTests: XCTestCase {
   }
 
   func testSymlinkInWorkspaceCXX() async throws {
-    // FIXME: should be possible to use InMemoryFileSystem.
     let fs = localFileSystem
     try await withTestScratchDir { tempDir in
       try fs.createFiles(
@@ -627,7 +617,6 @@ final class SwiftPMWorkspaceTests: XCTestCase {
   }
 
   func testSwiftDerivedSources() async throws {
-    // FIXME: should be possible to use InMemoryFileSystem.
     let fs = localFileSystem
     try await withTestScratchDir { tempDir in
       try fs.createFiles(