testsuite: explain why overriding function may fail

testsuite: explain why overriding function may fail

On my computer `testsuite/test-modprobe modprobe_install_cmd_loop` was
failing because when it forks off the shell the child process ends up
calling syscall() which are are supposed to wrap. Here's the backtrace:

	#0  0x00007ffff6fdb66b in raise () from /lib64/libc.so.6
	#1  0x00007ffff6fdd381 in abort () from /lib64/libc.so.6
	#2  0x00007ffff77bac97 in syscall (__sysno=<optimized out>)
	    at testsuite/init_module.c:362
	#3  0x00007fffef92d4e7 in hashmap_base_new.lto_priv () from /lib64/libnss_systemd.so.2
	#4  0x00007fffef953f50 in sd_bus_open_system () from /lib64/libnss_systemd.so.2
	#5  0x00007fffef943123 in _nss_systemd_getpwuid_r () from /lib64/libnss_systemd.so.2
	#6  0x00007ffff707eea5 in getpwuid_r@@GLIBC_2.2.5 () from /lib64/libc.so.6
	#7  0x00007ffff707e608 in getpwuid () from /lib64/libc.so.6
	#8  0x00005555555859e1 in get_current_user_info.part ()
	#9  0x00005555555a375a in initialize_shell_variables ()
	#10 0x0000555555580fde in shell_initialize ()
	#11 0x00005555555846ff in main ()<Paste>

The reason it fails on my system and not on, for e.g., a new one set up with
mkosi is that the call to getpwuid() depends on the contents
/etc/nsswitch.conf. The systemd module calls syscall() to implement gettid()
which we can't forward due to being a variadic function.

No fix is provided here, but at least it's explained why this happens.