Add U_TOPSRCDIR flag when compiling icu4c tests

Otherwise, cintltst.tsutil.ucptrietest.TrieTestSet1 fails with
the following error:
!!    Golden files for 'set1.16' differ at index 0; run cintltst with -G to write new goldens !
Golden files for 'set1.32' differ at index 0; run cintltst with -G to write new goldens !
Golden files for 'set1.8' differ at index 0; run cintltst with -G to write new goldens !
Golden files for 'set1.small16' differ at index 0; run cintltst with -G to write new goldens

Bug: 202948901
Test: atest CtsIcu4cTestCases
Change-Id: I6b99eb9224b3eff5997c55dc47f9acebea472e88
1 file changed