Relocate fuzzer files and the explorer_fuzzer rule. (#3017)

Putting fuzzer files under //testing to emphasize the testonly aspect
(consolidates bazel and common subdirectories). The attributes on
explorer_fuzzer are also a little skewed from what's desirable; it's
been working okay, but this should still be a refinement.

explorer/fuzzing/fuzzer_util.cpp

@@ -8,10 +8,10 @@
 
 #include "common/check.h"
 #include "common/error.h"
-#include "common/fuzzing/proto_to_carbon.h"
 #include "explorer/parse_and_execute/parse_and_execute.h"
 #include "llvm/Support/FileSystem.h"
 #include "llvm/Support/Path.h"
+#include "testing/fuzzing/proto_to_carbon.h"
 #include "tools/cpp/runfiles/runfiles.h"
 
 namespace Carbon::Testing {