test: check for CAP_SYS_PTRACE before testing for an attached debugger

If we don't have the capability to ptrace, we are probably running inside a
container, not the debugger. Check this first so we don't disable forking
mode.

Make this conditional on libcap - where libcap is not available always assume
we *do not* have a debugger attached. This is easier than telling everyone who
runs the tests in a confined system to install libcap.

Fixes #1285

test/meson.build

@@ -9,6 +9,7 @@ pwtest_deps = [
   pipewire_dep,
   mathlib,
   dl_lib,
+  cap_lib,
 ]
 
 pwtest_c_args = [