CI: drop Swift -Xfrontend solver-memory-threshold override from macOS/Linux tests

Simplify workflows to keep only -enable-testing and -DCI. Avoid unstable
compiler frontend flags in CI.

Author: krzyzanowskim

.github/workflows/linux.yml

@@ -21,4 +21,4 @@ jobs:
       run: swift build -v
 
     - name: Run tests
-      run: swift test -c release -Xswiftc -enable-testing -Xswiftc -DCI -Xswiftc -Xfrontend -Xswiftc -solver-memory-threshold -Xswiftc -Xfrontend -Xswiftc 999999999
+      run: swift test -c release -Xswiftc -enable-testing -Xswiftc -DCI

.github/workflows/macos.yml

@@ -26,4 +26,4 @@ jobs:
       run: swift build -v
 
     - name: Run tests
-      run: swift test -c release -Xswiftc -enable-testing -Xswiftc -DCI -Xswiftc -Xfrontend -Xswiftc -solver-memory-threshold -Xswiftc -Xfrontend -Xswiftc 999999999
+      run: swift test -c release -Xswiftc -enable-testing -Xswiftc -DCI