RFR (XS) 8186044: [TESTBUG] DumpSharedDictionary test sometimes fails in JPRT

[coleen.phillimore at oracle.com]

Summary: wrap test in CDSTestUtils.isUnableToMap(out)

bug link https://bugs.openjdk.java.net/browse/JDK-8186044
local webrev at 
http://oklahoma.us.oracle.com/~cphillim/webrev/8186044.01/webrev

Reran macos jprt testing, and local linux x64 testing.

thanks,
Coleen

(Ioi, is this right?)