There were three problems:
- Lack of a trailing space after prepending flags.
- Passing multiple flags in a string to --kubelet-flags seems to confuse
the flag parser; it stops parsing ALL flags as soon as it sees the
second kubelet flag. Fortunately, all instances of --kubelet-flags are
combined together, so we can just pass two of those.
- --feature-gates should be passed to the test framework, which then
forwards it to the kubelet, instead of using --kubelet-flags.

This hopefully fixes the dynamic config test failures on COS, which
started after #45602.