Remove LoadBTF and allow multiple btf files for testing

[ScriptSathi]

This issue is the follow-up to this conversation #3143 (comment)

The idea is to make a more simple use of the btfFile global variable in case we want to test multiple BTF files in a single test.

I think what we can do here is using a callback to set the btfFile, then perform the test, and global variable like this.

func RunTestOnSingleBtf(t *testing.T, btfFname string, test func(t *testing.T)) {
	if err := btfFileExists(btfFname); err != nil {
		t.Fatal(err)
	}
	btfFile = btfFname
        t.Run(btfFname, test)
	btfFile = ""
}

func TestFindBtfFuncParamFromHook(t *testing.T) {
	RunTestOnSingleBtf(t, defaults.DefaultBTFFile, testFindBtfFuncParamFromHook)
        RunTestOnSingleBtf(t, "/path/to/my/btf/2", testFindBtfFuncParamFromHook)
        RunTestOnSingleBtf(t, "/path/to/my/btf/3", testFindBtfFuncParamFromHook)
}

In FindBtfFuncParamFromHook we could also remove the use LoadBtf() by simply using NewBtf()

tetragon/pkg/btf/btf.go

Lines 118 to 120 in fc759c4

	func FindBtfFuncParamFromHook(hook string, argIndex int) (*btf.FuncParam, error) {
	var hookFn *btf.Func
	spec, err := LoadBTF()

Similarly we could refactor this below code by using such a function

tetragon/pkg/btf/validation_test.go

Lines 56 to 63 in fc759c4

	btfFname := filepath.Join(testdataPath, "btf", "vmlinux-5.4.104+")
	if _, err := os.Stat(btfFname); err != nil {
	t.Skipf("%s not found", btfFname)
	}
	btf, err := btf.LoadSpec(btfFname)
	if err != nil {
	t.Fatalf("failed to initialize BTF: %s", err)
	}

@kkourt, do you think it's close enough to what you were thinking of ?

[kkourt]

@kkourt, do you think it's close enough to what you were thinking of?
Yes, thanks for writing up!