Rename //explorer/common to base (#3103)

Renaming per #3100

explorer/base/error_builders_test.cpp

@@ -0,0 +1,26 @@
+// 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
+
+#include "explorer/base/error_builders.h"
+
+#include <gtest/gtest.h>
+
+#include "explorer/base/source_location.h"
+#include "testing/util/test_raw_ostream.h"
+
+namespace Carbon::Testing {
+namespace {
+
+TEST(ErrorBuildersTest, ProgramError) {
+  Error err = ProgramError(SourceLocation("x", 1, FileKind::Main)) << "test";
+  EXPECT_EQ(err.location(), "x:1");
+  EXPECT_EQ(err.message(), "test");
+
+  TestRawOstream out;
+  out << err;
+  EXPECT_EQ(out.TakeStr(), "x:1: test");
+}
+
+}  // namespace
+}  // namespace Carbon::Testing