kunit: add macro to allow conditionally exposing static symbols to tests

Create two macros:

VISIBLE_IF_KUNIT - A macro that sets symbols to be static if CONFIG_KUNIT
is not enabled. Otherwise if CONFIG_KUNIT is enabled there is no change to
the symbol definition.

EXPORT_SYMBOL_IF_KUNIT(symbol) - Exports symbol into
EXPORTED_FOR_KUNIT_TESTING namespace only if CONFIG_KUNIT is enabled. Must
use MODULE_IMPORT_NS(EXPORTED_FOR_KUNIT_TESTING) in test file in order to
use symbols.
Signed-off-by: Rae Moar <rmoar@google.com>
Reviewed-by: John Johansen <john.johansen@canonical.com>
Reviewed-by: David Gow <davidgow@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

include/kunit/visibility.h

+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * KUnit API to allow symbols to be conditionally visible during KUnit
+ * testing
+ *
+ * Copyright (C) 2022, Google LLC.
+ * Author: Rae Moar <rmoar@google.com>
+ */
+
+#ifndef _KUNIT_VISIBILITY_H
+#define _KUNIT_VISIBILITY_H
+
+#if IS_ENABLED(CONFIG_KUNIT)
+    /**
+     * VISIBLE_IF_KUNIT - A macro that sets symbols to be static if
+     * CONFIG_KUNIT is not enabled. Otherwise if CONFIG_KUNIT is enabled
+     * there is no change to the symbol definition.
+     */
+    #define VISIBLE_IF_KUNIT
+    /**
+     * EXPORT_SYMBOL_IF_KUNIT(symbol) - Exports symbol into
+     * EXPORTED_FOR_KUNIT_TESTING namespace only if CONFIG_KUNIT is
+     * enabled. Must use MODULE_IMPORT_NS(EXPORTED_FOR_KUNIT_TESTING)
+     * in test file in order to use symbols.
+     */
+    #define EXPORT_SYMBOL_IF_KUNIT(symbol) EXPORT_SYMBOL_NS(symbol, \
+	    EXPORTED_FOR_KUNIT_TESTING)
+#else
+    #define VISIBLE_IF_KUNIT static
+    #define EXPORT_SYMBOL_IF_KUNIT(symbol)
+#endif
+
+#endif /* _KUNIT_VISIBILITY_H */