fdo#74747: Write test for this first.

Change-Id: Ia528904405cbb698b9cf08d4a8531c60780e0d9e
fdo#74747: Test cached numeric results too just to be safe.

Change-Id: I00c68694859fdb9a2f31f50cb3c63984ed146146
fdo#74747: Make use of cached string formula results.

Just like we do with cached numeric formula results.

Change-Id: Ib8b311b540caeb47d8c2162a456f7490c5882ad5
fdo#74747: Correctly inspect formula result value for xlsx export.

Change-Id: I757a8eb371b432970885e2fbd6aea9dd965ab5c0