tests/docker: make _copy_with_mkdir accept missing files

tests/docker: make _copy_with_mkdir accept missing files

Depending on the linker/ldd setup we might get a file with no path.
Typically this is the psuedo library linux-vdso.so which doesn't
actually exist on the disk. Rather than try and catch these distro
specific edge cases just shout about it and try and continue.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Tested-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20210202134001.25738-4-alex.bennee@linaro.org>