Set number of test shards back to 7.

Difficult to say which is best, but something around 7-9 apparently.
Accumulated C/C++ compile times only ever go up with increasing shard count (tried 5, 7, 8, 9, 12) but the total test time is lowest at 7-9, depending on the specific run. So, let's waste less cycles overall.

Tools/ci-run.sh

@@ -97,4 +97,4 @@ python runtests.py \
    $LIMITED_API \
    $EXCLUDE \
    $(if [ "$COVERAGE" == "1" ]; then echo " --coverage"; fi) \
-   $(if [ -z "$TEST_CODE_STYLE" ]; then echo " -j8 "; fi)
+   $(if [ -z "$TEST_CODE_STYLE" ]; then echo " -j7 "; fi)