Add compose-symlinks testcase

This is a regression test for the issue raised here:
#270

tests/integration/compose-symlinks.py

+import io
+import os
+import sys
+import pytest
+
+from buildstream import _yaml
+
+from tests.testutils import cli_integration as cli
+from tests.testutils.integration import walk_dir
+
+
+pytestmark = pytest.mark.integration
+
+
+DATA_DIR = os.path.join(
+    os.path.dirname(os.path.realpath(__file__)),
+    "project"
+)
+
+
+# Test that staging a file inside a directory symlink works as expected.
+#
+# Regression test for https://gitlab.com/BuildStream/buildstream/issues/270
+@pytest.mark.datafiles(DATA_DIR)
+def test_compose_symlinks(cli, tmpdir, datafiles):
+    project = os.path.join(datafiles.dirname, datafiles.basename)
+    checkout = os.path.join(cli.directory, 'checkout')
+    element_path = os.path.join(project, 'elements')
+
+    # Symlinks do not survive being placed in a source distribution
+    # ('setup.py sdist'), so we have to create the one we need here.
+    project_files = os.path.join(project, 'files', 'compose-symlinks', 'base')
+    symlink_file = os.path.join(project_files, 'sbin')
+    os.symlink(os.path.join('usr', 'sbin'), symlink_file, target_is_directory=True)
+
+    result = cli.run(project=project, args=['build', 'compose-symlinks/compose.bst'])
+    result.assert_success()
+
+    result = cli.run(project=project, args=['checkout', 'compose-symlinks/compose.bst', checkout])
+    result.assert_success()
+
+    assert set(walk_dir(checkout)) == set(['/sbin', '/usr', '/usr/sbin',
+                                           '/usr/sbin/init', '/usr/sbin/dummy'])

tests/integration/project/elements/compose-symlinks/base.bst

+kind: import
+
+sources:
+- kind: local
+  path: files/compose-symlinks/base

tests/integration/project/elements/compose-symlinks/compose.bst

+kind: compose
+
+depends:
+- filename: compose-symlinks/base.bst
+  type: build
+- filename: compose-symlinks/overlay.bst
+  type: build
+
+config:
+  include:
+  - runtime

tests/integration/project/elements/compose-symlinks/overlay.bst

+kind: import
+
+sources:
+- kind: local
+  path: files/compose-symlinks/overlay

tests/integration/project/files/compose-symlinks/base/usr/sbin/dummy

+dummy

tests/integration/project/files/compose-symlinks/overlay/sbin/init

+foo