internal/app/shim: use syscall dispatcher

This enables instrumented testing of the shim.

Signed-off-by: Ophestra <cat@gensokyo.uk>

internal/app/app_test.go

@@ -574,10 +574,9 @@ func (s stubOsFileReadCloser) Stat() (fs.FileInfo, error) { panic("attempting to
 
 type stubNixOS struct {
 	usernameErr map[string]error
+	panicDispatcher
 }
 
-func (k *stubNixOS) new(func(k syscallDispatcher)) { panic("not implemented") }
-
 func (k *stubNixOS) getpid() int { return 0xdeadbeef }
 func (k *stubNixOS) getuid() int { return 1971 }
 func (k *stubNixOS) getgid() int { return 100 }