selftests/bpf: rename btf_decl_tag.c to test_btf_decl_tag.c

The uapi btf.h contains the following declaration:
struct btf_decl_tag {
__s32 component_idx;
};

The skeleton will also generate a struct with name
"btf_decl_tag" for bpf program btf_decl_tag.c.

Rename btf_decl_tag.c to test_btf_decl_tag.c so
the corresponding skeleton struct name becomes
"test_btf_decl_tag". This way, we could include
uapi btf.h in prog_tests/btf_tag.c.
There is no functionality change for this patch.

Signed-off-by: Yonghong Song <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Alexei Starovoitov <[e-mail]>

tools/testing/selftests/bpf/prog_tests/btf_tag.c

···
 // SPDX-License-Identifier: GPL-2.0
 /* Copyright (c) 2021 Facebook */
 #include <test_progs.h>
-#include "btf_decl_tag.skel.h"
+#include "test_btf_decl_tag.skel.h"
 
 /* struct btf_type_tag_test is referenced in btf_type_tag.skel.h */
 struct btf_type_tag_test {
···
 
 static void test_btf_decl_tag(void)
 {
-	struct btf_decl_tag *skel;
+	struct test_btf_decl_tag *skel;
 
-	skel = btf_decl_tag__open_and_load();
+	skel = test_btf_decl_tag__open_and_load();
 	if (!ASSERT_OK_PTR(skel, "btf_decl_tag"))
 		return;
 
···
 		test__skip();
 	}
 
-	btf_decl_tag__destroy(skel);
+	test_btf_decl_tag__destroy(skel);
 }
 
 static void test_btf_type_tag(void)