Fix some tests that construct SolidityConfig objects manually

Author: alcuadrado

packages/hardhat/test/internal/builtin-plugins/solidity/build-system/build-system.ts

@@ -98,6 +98,7 @@ describe(
       },
       npmFilesToBuild: [],
       registeredCompilerTypes: ["solc"],
+      splitTestsCompilation: false,
     };
 
     before(async () => {

packages/hardhat/test/internal/builtin-plugins/solidity/config.ts

@@ -1764,6 +1764,7 @@ describe("validateResolvedConfig", () => {
         profiles,
         npmFilesToBuild: [],
         registeredCompilerTypes,
+        splitTestsCompilation: false,
       },
     }) as unknown as HardhatConfig;