Provide local paths for file tests. (#2830)

The intent of this change is that instead of paths looking like `explorer/testdata/foo/bar.carbon` (repo-relative), they're now just `bar.carbon` (local). The consequence is that paths should be a bit more durable in various environments, and just paths should be shorter and easier to read.

The explorer's prelude is an exception to this since it comes from data, rather than being the test target. Due to the change in approaches, it needs the regex again.

Uses #2829

testing/file_test/file_test_base_test.cpp

@@ -17,11 +17,13 @@ namespace {
 
 class FileTestBaseTest : public FileTestBase {
  public:
-  explicit FileTestBaseTest(llvm::StringRef path) : FileTestBase(path) {}
+  explicit FileTestBaseTest(const std::filesystem::path& path)
+      : FileTestBase(path) {}
 
   auto RunOverFile(llvm::raw_ostream& stdout, llvm::raw_ostream& stderr)
       -> bool override {
-    if (filename() == "example.carbon") {
+    auto filename = path().filename();
+    if (filename == "example.carbon") {
       stdout << "something\n"
                 "\n"
                 "8: Line delta\n"
@@ -29,11 +31,11 @@ class FileTestBaseTest : public FileTestBase {
                 "+*[]{}\n"
                 "Foo baz\n";
       return true;
-    } else if (filename() == "fail_example.carbon") {
+    } else if (filename == "fail_example.carbon") {
       stderr << "Oops\n";
       return false;
     } else {
-      ADD_FAILURE() << "Unexpected file: " << path().str();
+      ADD_FAILURE() << "Unexpected file: " << filename;
       return false;
     }
   }
@@ -41,10 +43,12 @@ class FileTestBaseTest : public FileTestBase {
 
 }  // namespace
 
-auto RegisterFileTests(const std::vector<llvm::StringRef>& paths) -> void {
-  FileTestBaseTest::RegisterTests(
-      "FileTestBaseTest", paths,
-      [](llvm::StringRef path) { return new FileTestBaseTest(path); });
+auto RegisterFileTests(const std::vector<std::filesystem::path>& paths)
+    -> void {
+  FileTestBaseTest::RegisterTests("FileTestBaseTest", paths,
+                                  [](const std::filesystem::path& path) {
+                                    return new FileTestBaseTest(path);
+                                  });
 }
 
 }  // namespace Carbon::Testing