Support NODE_ICU_DATA in forked processes

[jgkim]

With the almost same reason with #531, we need to ensure NODE_ICU_DATA is absolute in lib/fork.js.

Error Message & Stack Trace

#
# Fatal error in , line 0
# Failed to create ICU date format, are ICU data files missing?
#

==== C stack trace ===============================

 1: V8_Fatal
 2: v8::internal::(anonymous namespace)::CreateICUDateFormat(v8::internal::Isolate*, icu_57::Locale const&, v8::internal::Handle<v8::internal::JSObject>)
 3: v8::internal::Runtime_CreateDateTimeFormat(int, v8::internal::Object**, v8::internal::Isolate*)
 4: 0x1e1d30b0961b
 5: 0x1e1d30ced87c

[jgkim]

It seems like this bug is also caused by #32.

[novemberborn]

@jgkim #32 changes the CWD to always be that of the nearest package.json. However if you were to run AVA from a different directory, with a relative NODE_ICU_DATA path, the path in the child processes will still be wrong.

In other words we still need to make the path absolute.