Reorganize tests into dirs and rename numbered tests (#825)

There's a small update to update_checks.py to handle the recursive directories. Also, I'm only using one level of nesting in this PR but really no reason we can't do more. I'm just not sure what clustering is best right now.

As a pattern, I'm trying to name all failing tests `fail_*.carbon`.

executable_semantics/update_checks.py

@@ -24,30 +24,28 @@ _AUTOUPDATE_MARKER = "// AUTOUPDATE: executable_semantics %s\n"
 def _get_tests():
     """Get the list of tests from the filesystem."""
     tests = set()
-    for path in list(os.listdir(_TESTDATA)):
-        f = os.path.basename(path)
-        if f == "lit.cfg":
-            # Ignore the lit config.
-            continue
-        basename, ext = os.path.splitext(f)
-        if ext == ".carbon":
-            tests.add(basename)
-        else:
-            sys.exit("Unrecognized file type in testdata: %s" % f)
+    for root, _, files in os.walk(_TESTDATA):
+        for f in files:
+            if f == "lit.cfg":
+                # Ignore the lit config.
+                continue
+            if os.path.splitext(f)[1] == ".carbon":
+                tests.add(os.path.join(root, f))
+            else:
+                sys.exit("Unrecognized file type in testdata: %s" % f)
     return tests
 
 
 def _update_check(test):
     """Updates the CHECK: lines for `test` by running executable_semantics."""
-    test_file = "%s/%s.carbon" % (_TESTDATA, test)
-    with open(test_file) as f:
+    with open(test) as f:
         orig_lines = f.readlines()
     if _AUTOUPDATE_MARKER not in orig_lines:
-        raise ValueError("No autoupdate marker in %s" % test_file)
+        raise ValueError("No autoupdate marker in %s" % test)
     # Run executable_semantics to general output.
     # (`bazel run` would serialize)
     p = subprocess.run(
-        ["%s/executable_semantics" % _BINDIR, test_file],
+        ["%s/executable_semantics" % _BINDIR, test],
         stdout=subprocess.PIPE,
         stderr=subprocess.STDOUT,
     )
@@ -57,7 +55,7 @@ def _update_check(test):
     # when used.
     # TODO: Maybe revisit and see if lit can be convinced to give a
     # root-relative path.
-    out = out.replace(test_file, "{{.*}}/%s.carbon" % test)
+    out = out.replace(test, "{{.*}}/%s" % test)
 
     # Remove old OUT.
     lines_without_check = [
@@ -65,7 +63,7 @@ def _update_check(test):
     ]
     autoupdate_index = lines_without_check.index(_AUTOUPDATE_MARKER)
     assert autoupdate_index >= 0
-    with open(test_file, "w") as f:
+    with open(test, "w") as f:
         f.writelines(lines_without_check[: autoupdate_index + 1])
         f.writelines(["// CHECK: %s\n" % x for x in out.splitlines()])
         f.writelines(lines_without_check[autoupdate_index + 1 :])