[RFR]: Suppress some String races

[Arthur Eubanks]

Webrev: http://cr.openjdk.java.net/~aeubanks/tsanstring/webrev.00/index.html

Some common String operations are racy, e.g. hashCode or concatenation.
These races should be suppressed, since they are in the JDK.
This change adds a test that calls hashCode() on a const String and does
local String concatenation.