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";
+// ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ @NotFound
+
+contract MyContract
+{
+}
+// ----
+// using-custom-includes: @NotFound 6275

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

@@ -0,0 +1,7 @@
+// SPDX-License-Identifier: UNLICENSED
+pragma solidity >=0.8.0;
+
+library OtherLib
+{
+    function f(uint n) public returns (uint) { return n + 1; }
+}