elf: skip BenchmarkWriteELF if ELF file wasn't built

When running `go test -run=^$ -bench=. ./...` on the Cilium source,
BenchmarkWriteELF currently fails with:

  --- FAIL: BenchmarkWriteELF
        elf_test.go:201: failed to open ELF file ../../test/bpf/elf-demo.o: open ../../test/bpf/elf-demo.o: no such file or directory

That particular file is only built when running tests/benchmarks using
the Makefile-based flow. In order to still allow Cilium's benchmarks
suite to run without errors when invoked using native `go test -bench`,
just skip BenchmarkWriteELF if the ELF file doesn't exist.

In the future we might want to embed that ELF file into the
test/benchmark itself using bpf2go [1].

[1] https://pkg.go.dev/github.com/cilium/ebpf/cmd/bpf2go

For #17535

Reported-by: Emmanuel T Odeke <emmanuel@orijtech.com>
Signed-off-by: Tobias Klauser <tobias@cilium.io>

pkg/elf/elf_test.go

@@ -9,8 +9,10 @@ package elf
 import (
 	"bytes"
 	"crypto/sha256"
+	"errors"
 	"fmt"
 	"io"
+	"io/fs"
 	"os"
 	"path/filepath"
 	"testing"
@@ -197,7 +199,11 @@ func BenchmarkWriteELF(b *testing.B) {
 	defer os.RemoveAll(tmpDir)
 
 	elf, err := Open(baseObjPath)
-	if err != nil {
+	if errors.Is(err, fs.ErrNotExist) {
+		// If the ELF file couldn't be found most likely it
+		// wasn't built. See https://github.com/cilium/cilium/issues/17535
+		b.Skip("ELF file not found, skipping benchmark")
+	} else if err != nil {
 		b.Fatal(err)
 	}
 	defer elf.Close()