JDK 10 RFR of JDK-8181394: Refactor shell test java/nio/file/Files/walkFileTree/find.sh to java

[Amy Lu]

java/nio/file/Files/walkFileTree/find.sh

Please review this patch to refactor the shell test to java.

bug: https://bugs.openjdk.java.net/browse/JDK-8181394
webrev: http://cr.openjdk.java.net/~amlu/8181394/webrev.00/

(I'll add @build jdk.test.lib.process.* if this is the "best" workaround 
of CODETOOLS-7901986)

Thanks,
Amy