Move these external reference test to ucalc_formula.cxx.

Change-Id: Iaf8fc14e413eb44de3c796c97112e0868ad672df
fdo#72041: Add test for this.

Change-Id: I3d0f692529c3425aaab6080818882e8fb5326d1b
fdo#72041: Intern strings in the external ref cache with the host document.

To ensure that string comparison with those from the external ref cache
works correctly in functions such as VLOOKUP.

Change-Id: Ib5a466cb6c4b26439abe61b0c17406fc8139f6c0