El Capitan: DNS cache won't flush
You are mixing a lot of completely or partly different/unrelated things here.


To properly and quickly name resolve your hard disk/SSD, you also have to add its name to the hosts file:

127.0.0.1    localhost santas-macbook-pro-2.local tt-this_is_a_VM_volume_and_therefore_slower_than_a_real_disk 


Now you can check the response time by pinging it:

ping tt-this_is_a_VM_disk_and_therefore_slower_than_a_real_disk 
PING tt-this_is_a_VM_disk_and_therefore_slower_than_a_real_disk (127.0.0.1): 56 data bytes
64 bytes from 127.0.0.1: icmp_seq=0 ttl=64 time=0.054 ms
64 bytes from 127.0.0.1: icmp_seq=1 ttl=64 time=0.089 ms
64 bytes from 127.0.0.1: icmp_seq=2 ttl=64 time=0.071 ms

--- tt-this_is_a_VM_disk_and_therefore_slower_than_a_real_disk ping statistics ---
3 packets transmitted, 3 packets received, 0.0% packet loss
round-trip min/avg/max/stddev = 0.054/0.071/0.089/0.014 ms


The average round-trip time is ~0.071 ms. So the time needed to resolve it is less than ~0.071/2 ms. With a real HDD/SSD you can expect even faster responses.

Please use the short tt (the name of your volume) instead of my long tt-this_.... name to get an even quicker response.

To check your DNS cache open Console.app (enter mDNSResponder in the Search field) and Terminal.app.

Check the time and enter in Terminal: sudo killall -INFO mDNSResponder

Roughly the first half of the output in Console with the time stamp noted previously is the DNS cache. The cache part starts with ------------ Cache ------------- and ends with a summary like Cache currently contains 391 entities; 33 referenced by active questions.

To kill the cache use sudo dscacheutil -flushcache; sudo killall -HUP mDNSResponder

Re-check the cache by entering  sudo killall -INFO mDNSResponder again.
discoveryutil was introduced in OS X 10.10.0 and replaced by its predecessor mDNSResponder in OS X 10.10.4.
To modify your local hostname and computer name use scutil:

sudo scutil --set LocalHostName trial
sudo scutil --set ComputerName Trial


If you still get a Bonjour name (e.g. Santa's MacBook Pro.local) your hostname is probably set incorrectly. Either change the hostname by entering:

sudo scutil --set HostName trial


or remove it completely (recommended) by modifying the file preferences.list:

sudo  nano /Library/Preferences/SystemConfiguration/preferences.plist


and remove your current malformed HostName near the bottom of the file:

    &lt;key&gt;HostName&lt;/key&gt;
    &lt;string&gt;Santa's MacBook Pro.local&lt;/string&gt;


☛

    &lt;key&gt;HostName&lt;/key&gt;
    &lt;string&gt;&lt;/string&gt;


You may also have to remove the preferences backup file preferences.plist.old. Reboot afterwards.
Name resolution is roughly done in this order: 

  files (e.g. /etc/hosts) &gt; mDNS &gt; DNS)  \
   ↕︎                         (↓ )   (↓)   | rather remote hostnames
DNS cache (memory) (←-----------------)  /
   ↑                                       \
memory                                      | rather local hostnames (including hosts file)
   ↑                                        |
 file (preferences.plist)                  /

inetTester is a tool to quickly check the elapsed time spent from the Java layer to access the localhost domain name. This is not the time to get (resolve) the local hostname. The hostname is stored somewhere in memory and a system call takes nanoseconds to get it.
The name of your main volume (previously Santa's MacBook, now tt) is in no way related with the DNS cache or a slow local hostname resolution.




Regarding item 1 in the list. I wear Dark Vader's helmet party hat and got nasty temporarily.