tests/tcg: move test plugins into tcg subdir

You cannot use plugins without TCG enabled so it doesn't make sense to
have them separated off in the test directory structure. While we are
at it rename the directory to plugins to reflect the plural nature of
the directory and match up with contrib/plugins.

Suggested-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20240729144414.830369-10-alex.bennee@linaro.org>

tests/tcg/plugins/empty.c

@@ -0,0 +1,32 @@
+/*
+ * Copyright (C) 2018, Emilio G. Cota <cota@braap.org>
+ *
+ * License: GNU GPL, version 2 or later.
+ *   See the COPYING file in the top-level directory.
+ */
+#include <inttypes.h>
+#include <assert.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+#include <stdio.h>
+
+#include <qemu-plugin.h>
+
+QEMU_PLUGIN_EXPORT int qemu_plugin_version = QEMU_PLUGIN_VERSION;
+
+/*
+ * Empty TB translation callback.
+ * This allows us to measure the overhead of injecting and then
+ * removing empty instrumentation.
+ */
+static void vcpu_tb_trans(qemu_plugin_id_t id, struct qemu_plugin_tb *tb)
+{ }
+
+QEMU_PLUGIN_EXPORT int qemu_plugin_install(qemu_plugin_id_t id,
+                                           const qemu_info_t *info,
+                                           int argc, char **argv)
+{
+    qemu_plugin_register_vcpu_tb_trans_cb(id, vcpu_tb_trans);
+    return 0;
+}