UT 2. add ut cases for libntp/authreadkeys.c

Two more UT cases for authreadkeys.
1. Config file doesn't exist.
2. Config file exist and there is only one config in it.
UT 3. modify ut cases for libntp/authreadkeys.c

Create the basic config file only when it is needed.
UT 4. add ut cases for libntp/emalloc.c

Create test case for estrdup_impl in non-debug mode.