Android patch: Deduplicate data finding code in tests.

The code to find the data and testdata directories was needlessly
duplicated across cintltst and intltest, causing pain for porting to
other platforms. Extract all this functionality into ctestfw so it
can be shared across the test suites.

The upstream request is tracked at
https://unicode-org.atlassian.net/browse/ICU-13849.

Bug: 120776993
Test: treehugger
Change-Id: Ib5e69af3921e3c6401e2f975b640b34ed6593012
8 files changed