Test basic non-empty sdist creation

tests/sdist/create/library.dds

+Name: foo

tests/sdist/create/package.dds

+Name: foo
+Version: 1.2.3

tests/sdist/sdist_test.py

+from tests.dds import DDS, dds_fixture_conf_1
+
+@dds_fixture_conf_1('create')
+def test_create_sdist(dds: DDS):
+    dds.sdist_create()
+    sd_dir = dds.build_dir / 'created-sdist.sds'
+    assert sd_dir.is_dir()
+    foo_cpp = sd_dir / 'src/foo.cpp'
+    assert foo_cpp.is_file()
+    header_hpp = sd_dir / 'include/header.hpp'
+    assert header_hpp.is_file()
+    header_h = sd_dir / 'include/header.h'
+    assert header_h.is_file()