tests: Avoid modifying '17 prebuild shared' test dir

Tests can tread on each other's toes when parallelism is high enough.

In this case, `test_prebuilt_shared_lib` creates an object file in the
`17 prebuilt shared` test dir.

`test_prebuilt_shared_lib_rpath_same_prefix` uses `shutil.copytree` to
copy that same test dir to a temporary location.

If the former test cleans up its object file while `copytree` is
running, the copy can fail with a fatal ENOENT `shutil.Error`.

Use `copy_srcdir` to prevent this from happening.

unittests/allplatformstests.py

@@ -1766,7 +1766,7 @@ def build_shared_lib(self, compiler, source, objectfile, outfile, impfile, extra
 
     def test_prebuilt_shared_lib(self):
         (cc, _, object_suffix, shared_suffix) = self.detect_prebuild_env()
-        tdir = os.path.join(self.unit_test_dir, '17 prebuilt shared')
+        tdir = self.copy_srcdir(os.path.join(self.unit_test_dir, '17 prebuilt shared'))
         source = os.path.join(tdir, 'alexandria.c')
         objectfile = os.path.join(tdir, 'alexandria.' + object_suffix)
         impfile = os.path.join(tdir, 'alexandria.lib')