tools/bootconfig: Support mixed value and subkey test cases

Update test case to support mixed value and subkey on a key.

Link: https://lkml.kernel.org/r/162262195568.264090.12431204030021242896.stgit@devnote2

Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>