RFR: JDK-8196875 Update run-test instructions for TEST_MODE

[Magnus Ihse Bursie]

As reported by Martin Buchholz <marti...@google.com>:
#### TEST_MODE
The test mode (`-agentvm`, `-samevm` or `-othervm`).

but TEST_MODE=-agentvm is rejected - it must be TEST_MODE=agentvm.

and Jon Gibbons noted:
.. and samevm mode no longer exists.

Bug: https://bugs.openjdk.java.net/browse/JDK-8196875
WebRev: 
http://cr.openjdk.java.net/~ihse/JDK-8196875-update-test-readme-for-TEST_MODE/webrev.01