test: Pick platform randomly in "sloppy" mode.

This will give us more coverage of android when we're running tools/test
in sloppy mode (which is the default).

This does make things somewhat flaky, but on the whole it makes us more
likely to notice failures that only occur on one platform sooner.

See prior discussion:
#4700 (comment)

tools/test

@@ -216,15 +216,18 @@ run_jest() {
             ;;
     esac
 
+    PLATFORMS=("ios" "android")
+    RANDOM_PLATFORM=${PLATFORMS[RANDOM%2]}
+
     case "$platform" in
         ios) jest_args+=( --selectProjects ios );;
         android) jest_args+=( --selectProjects android );;
         both) jest_args+=( --selectProjects ios android );;
 
-        # This is where `sloppy` is sloppy: we just choose iOS, so the
-        # tests will run faster, but at the expense of a (relatively
-        # small) loss of coverage.
-        sloppy) jest_args+=( --selectProjects ios );;
+        # This is where `sloppy` is sloppy: we choose a platform randomly, so
+        # the tests will run faster, but at the expense of a (relatively small)
+        # loss of coverage.
+        sloppy) jest_args+=( --selectProjects "$RANDOM_PLATFORM");;
     esac
 
     check_node || return