Update tuple tests, and merge in expr-category (carbon-language#5549)

In the vein of carbon-language#5455.

This merges the one expr_category test into tuple testing because it
seemed closely associated (particularly with
in_place_tuple_init.carbon), and it didn't seem worth keeping a
directory for a single test.

Note the entire "access" directory is combined into
element_access.carbon.

Author: jonmeow