prog: recognize ENOTSUPP in testRun()

Not all program types support PROG_TEST_RUN on all kernel versions.
Previously, when testing a program type that didn't have a test runner
implemented, the following error would be returned:

  Error when running: can't test program: can't run test: errno 524

Since this isn't overly descriptive, make it clear to the caller that
the kernel supports test runs, just not for the particular program type.

Signed-off-by: Timo Beckers <timo@isovalent.com>

prog.go

@@ -585,7 +585,9 @@ func (p *Program) Benchmark(in []byte, repeat int, reset func()) (uint32, time.D
 
 var haveProgTestRun = internal.FeatureTest("BPF_PROG_TEST_RUN", "4.12", func() error {
 	prog, err := NewProgram(&ProgramSpec{
-		Type: SocketFilter,
+		// The first PROG_TEST_RUN patches shipped in 4.12 only supported testing
+		// SKB and XDP programs.
+		Type: SchedACT,
 		Instructions: asm.Instructions{
 			asm.LoadImm(asm.R0, 0, asm.DWord),
 			asm.Return(),
@@ -665,6 +667,10 @@ func (p *Program) testRun(in []byte, repeat int, reset func()) (uint32, []byte,
 			continue
 		}
 
+		if errors.Is(err, unix.ENOTSUPP) {
+			return 0, nil, 0, fmt.Errorf("can't test program type %s: %w", p.Type(), ErrNotSupported)
+		}
+
 		return 0, nil, 0, fmt.Errorf("can't run test: %w", err)
 	}