lsp: Adding test for custom include paths.

test/libsolidity/lsp/include-paths/using-custom-includes.sol

@@ -0,0 +1,11 @@
+// SPDX-License-Identifier: UNLICENSED
+pragma solidity >=0.8.0;
+
+   import "otherlib/otherlib.sol";
+// ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ @SomeTag
+
+contract MyContract
+{
+}
+// ----
+// using-custom-includes: @SomeTag 6275

test/libsolidity/lsp/other-include-dir/otherlib/otherlib.sol

@@ -0,0 +1,6 @@
+// SPDX-License-Identifier: UNLICENSED
+pragma solidity >=0.8.0;
+
+library MyRootLib
+{
+}

test/lsp.py

@@ -16,6 +16,7 @@
 from itertools import islice
 from pathlib import PurePath
 from typing import Any, List, Optional, Tuple, Union, NewType
+from pprint import pprint
 
 import colorama  # Enables the use of SGR & CUP terminal VT sequences on Windows.
 from deepdiff import DeepDiff
@@ -1381,6 +1382,37 @@ def get_test_tags(self, test_name: TestName, sub_dir=None, verbose=False):
         return markers
 
 
+    def test_custom_includes(self, solc: JsonRpcProcess) -> None:
+        self.setup_lsp(solc, expose_project_root=False)
+        solc.send_notification(
+            'workspace/didChangeConfiguration',
+            {
+                'settings':
+                {
+                    'include-paths':
+                    [
+                        f"{self.project_root_dir}/other-include-dir"
+                    ]
+                }
+            }
+        )
+        published_diagnostics = self.open_file_and_wait_for_diagnostics(solc, 'include-paths/using-custom-includes')
+
+        pprint(published_diagnostics)
+        self.expect_equal(len(published_diagnostics), 2, "Diagnostic reports for 2 files")
+
+        # test file
+        report = published_diagnostics[0]
+        self.expect_equal(report['uri'], self.get_test_file_uri('using-custom-includes', 'include-paths'))
+        diagnostics = report['diagnostics']
+        self.expect_equal(len(diagnostics), 0, "no diagnostics")
+
+        # imported file
+        report = published_diagnostics[1]
+        self.expect_equal(report['uri'], f"{self.project_root_uri}/other-include-dir/otherlib/otherlib.sol")
+        diagnostics = report['diagnostics']
+        self.expect_equal(len(diagnostics), 0, "no diagnostics")
+
     def test_didChange_in_A_causing_error_in_B(self, solc: JsonRpcProcess) -> None:
         # Reusing another test but now change some file that generates an error in the other.
         self.test_textDocument_didOpen_with_relative_import(solc)