Make the file_test binary work without custom environment variables. (#5442)

Instead of crashing when run outside of `bazel`, make the toolchain's
`file_test` binary work properly when no test-specific environment
variables are set. This makes it a lot easier to run `file_test` under a
debugger.

There are two main changes here:

- Don't crash if `$TEST_TMPDIR` is unset. Instead, fall back to LLVM's
temporary directory (typically `$TMPDIR`). We already did this in some
places in tests. We now do it in more places.
- Don't fall back to a target label of `<target>` in the reproduction
commands if `$TEST_TARGET` is unset, because this causes all the tests
to fail because their output doesn't match the expected output due to a
differing bazel run command. Instead explicitly specify the target from
the `FileTestBase`-derived class.

Infrastructure for this has been added generally, but only rolled out to
the toolchain `file_test` binary for now.

---------

Co-authored-by: Jon Ross-Perkins <jperkins@google.com>

toolchain/testing/file_test.cpp

@@ -62,6 +62,12 @@ class ToolchainFileTest : public FileTestBase {
     return component_ != "language_server";
   }
 
+  // Force a fixed bazel label to avoid spurious test failures due to differing
+  // run commands when running file_test binary outside of bazel.
+  auto GetBazelLabel() -> std::string override {
+    return "//toolchain/testing:file_test";
+  }
+
  private:
   // Controls whether `Run()` includes the prelude.
   auto is_no_prelude() const -> bool {