Update name of gs_update_ahr() test file to follow convention

[jdblischak]

Follow up to #378

I noticed that the test file for gs_update_ahr() was the only one that uses dashes instead of underscores in the filename.

$ ls tests/testthat/ -1 | grep -v '_'
fixtures/
helper.R
helper-developer-summary.R
helper-double-programming-ppwe.R
test-developer-ahr.R
test-developer-fastlag.R
test-developer-gridpts.R
test-developer-summary.R
test-independent-ahr.R
test-independent-gridpts.R
test-independent-gs-update-ahr.R
test-independent-h1.R
test-independent-hupdate.R
test-independent-rmst.R

[LittleBeannie]

Nice catch, thanks, @jdblischak !