Refactor lit testing to generate per-file tests. (#1063)

This switches to generating per-file targets such as `//executable_semantics/testdata:tuple/equality_false.carbon.test`, instead of one test covering all files. Essentially this relies on bazel instead of lit to handle test parallelization.

executable_semantics/testdata/BUILD

@@ -0,0 +1,15 @@
+# Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+# Exceptions. See /LICENSE for license information.
+# SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+load("//bazel/testing:lit.bzl", "glob_lit_tests")
+
+glob_lit_tests(
+    data = [
+        "//executable_semantics",
+        "@llvm-project//llvm:FileCheck",
+        "@llvm-project//llvm:not",
+    ],
+    driver = "lit.cfg.py",
+    test_file_exts = ["carbon"],
+)