Migrate explorer tests to file_test and remove lit support. (#3050)

This migrates explorer tests to file_test, using the new --autoupdate
functionality. Per discussion, the trace tests that were using "not"
output are mostly migrated to checking full output. The main exception
is tests that were including trace output from the prelude: the prelude
output is pretty long (multiple MB already) and I think it wasn't the
intent to include, only trace output from the small program.

This is the remaining use of lit support, so the supporting libraries
are also removed here.

explorer/file_test.cpp

@@ -4,6 +4,7 @@
 
 #include "absl/flags/flag.h"
 #include "explorer/main.h"
+#include "re2/re2.h"
 #include "testing/file_test/file_test_base.h"
 #include "testing/util/test_raw_ostream.h"
 
@@ -17,7 +18,13 @@ namespace {
 
 class ExplorerFileTest : public FileTestBase {
  public:
-  using FileTestBase::FileTestBase;
+  explicit ExplorerFileTest(std::filesystem::path path)
+      : FileTestBase(std::move(path)),
+        prelude_line_re_(R"(prelude.carbon:(\d+))"),
+        timing_re_(R"((Time elapsed in \w+: )\d+(ms))") {
+    CARBON_CHECK(prelude_line_re_.ok()) << prelude_line_re_.error();
+    CARBON_CHECK(timing_re_.ok()) << timing_re_.error();
+  }
 
   auto Run(const llvm::SmallVector<llvm::StringRef>& test_args,
            const llvm::SmallVector<TestFile>& test_files,
@@ -52,13 +59,9 @@ class ExplorerFileTest : public FileTestBase {
       args.push_back(arg.data());
     }
 
-    // Trace output is only checked for a few tests.
-    bool check_trace_output =
-        path().string().find("trace_testdata/") != std::string::npos;
-
     int exit_code = ExplorerMain(
         args.size(), args.data(), /*install_path=*/"", PreludePath, stdout,
-        stderr, check_trace_output ? stdout : trace_stream_, fs);
+        stderr, check_trace_output() ? stdout : trace_stream_, fs);
 
     return exit_code == EXIT_SUCCESS;
   }
@@ -83,7 +86,37 @@ class ExplorerFileTest : public FileTestBase {
     return args;
   }
 
+  auto GetLineNumberReplacement(llvm::ArrayRef<llvm::StringRef> filenames)
+      -> LineNumberReplacement override {
+    if (check_trace_output()) {
+      return {.has_file = false,
+              .pattern = R"((DO NOT MATCH))",
+              // The `{{{{` becomes `{{`.
+              .line_formatv = "{{{{ *}}{0}"};
+    }
+    return FileTestBase::GetLineNumberReplacement(filenames);
+  }
+
+  auto DoExtraCheckReplacements(std::string& check_line) -> void override {
+    // Ignore the resulting column of EndOfFile because it's often the end of
+    // the CHECK comment.
+    RE2::GlobalReplace(&check_line, prelude_line_re_,
+                       R"(prelude.carbon:{{\\d+}})");
+    if (check_trace_output()) {
+      // Replace timings in trace output.
+      RE2::GlobalReplace(&check_line, timing_re_, R"(\1{{\\d+}}\2)");
+    }
+  }
+
+ private:
+  // Trace output is directly checked for a few tests.
+  auto check_trace_output() -> bool {
+    return path().string().find("/trace/") != std::string::npos;
+  }
+
   TestRawOstream trace_stream_;
+  RE2 prelude_line_re_;
+  RE2 timing_re_;
 };
 
 }  // namespace