Base: Add extra sync to run-tests-and-shutdown.sh

This shouldn't be necessary but reproducing the odd corrupted profile
data from the nightly Azure runs is proving tricky locally. Hopefully
this will mitigate the issue.

Base/home/anon/Tests/run-tests-and-shutdown.sh

@@ -13,6 +13,7 @@ unset LLVM_PROFILE_FILE
 echo "Failed: $fail_count" > ./test-results.log
 
 if test $DO_SHUTDOWN_AFTER_TESTS {
+    sync
     shutdown -n
 }