Disable flakey diagnostics tests in Swift 5.6 and below (swiftlang#1127)

It appears there is an issue in Swift 5.6 where a line of diagnostics
output is improperly cleared from the terminal by the build progress
prints. When this happens the tests potentially miss diagnostics they
expect to see. This only happens on Swift 5.6, so to ensure these tests
pass consistently, skip them there.

Author: plemarquand

test/integration-tests/DiagnosticsManager.test.ts

@@ -216,7 +216,14 @@ suite("DiagnosticsManager Test Suite", async function () {
                 assertHasDiagnostic(funcUri, expectedFuncErrorDiagnostic);
             }).timeout(2 * 60 * 1000); // Allow 2 minutes to build
 
-            test("Parses C diagnostics", async () => {
+            test("Parses C diagnostics", async function () {
+                const swiftVersion = workspaceContext.toolchain.swiftVersion;
+                // SPM will sometimes improperly clear diagnostics from the terminal, leading
+                // to a flakey test.
+                if (swiftVersion.isLessThan(new Version(5, 7, 0))) {
+                    this.skip();
+                }
+
                 await swiftConfig.update("diagnosticsStyle", "llvm");
                 const task = createBuildAllTask(cFolderContext);
                 // Run actual task
@@ -243,7 +250,14 @@ suite("DiagnosticsManager Test Suite", async function () {
                 assertHasDiagnostic(cUri, expectedDiagnostic2);
             });
 
-            test("Parses C++ diagnostics", async () => {
+            test("Parses C++ diagnostics", async function () {
+                const swiftVersion = workspaceContext.toolchain.swiftVersion;
+                // SPM will sometimes improperly clear diagnostics from the terminal, leading
+                // to a flakey test.
+                if (swiftVersion.isLessThan(new Version(5, 7, 0))) {
+                    this.skip();
+                }
+
                 await swiftConfig.update("diagnosticsStyle", "llvm");
                 const task = createBuildAllTask(cppFolderContext);
                 // Run actual task