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

toolchain/parser/parse_tree_file_test.cpp

@@ -17,21 +17,25 @@ namespace {
 
 class ParserFileTest : public FileTestBase {
  public:
-  explicit ParserFileTest(llvm::StringRef path) : FileTestBase(path) {}
+  explicit ParserFileTest(const std::filesystem::path& path)
+      : FileTestBase(path) {}
 
   auto RunOverFile(llvm::raw_ostream& stdout, llvm::raw_ostream& stderr)
       -> bool override {
     Driver driver(stdout, stderr);
-    return driver.RunFullCommand({"dump", "parse-tree", path()});
+    return driver.RunFullCommand(
+        {"dump", "parse-tree", path().filename().string()});
   }
 };
 
 }  // namespace
 
-auto RegisterFileTests(const std::vector<llvm::StringRef>& paths) -> void {
-  ParserFileTest::RegisterTests(
-      "ParserFileTest", paths,
-      [](llvm::StringRef path) { return new ParserFileTest(path); });
+auto RegisterFileTests(const std::vector<std::filesystem::path>& paths)
+    -> void {
+  ParserFileTest::RegisterTests("ParserFileTest", paths,
+                                [](const std::filesystem::path& path) {
+                                  return new ParserFileTest(path);
+                                });
 }
 
 }  // namespace Carbon::Testing