Skip motifbreakR test on macOS: parallel SIGPIPE crash in GHA

The MOTIFBREAKR test triggers a SIGPIPE/connection error in parallel
processing on macOS GHA runners. Skip on macOS and also add
skip_if_not_installed guards for motifbreakR and BSgenome.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: bschilder

tests/testthat/test-motifbreakR.R

@@ -1,6 +1,9 @@
 test_that("MOTIFBREAKR works", {
 
     testthat::skip_on_os("windows")
+    testthat::skip_on_os("mac")
+    testthat::skip_if_not_installed("motifbreakR")
+    testthat::skip_if_not_installed("BSgenome.Hsapiens.UCSC.hg19")
     library(BSgenome) ## <-- IMPORTANT!
     set.seed(2022)
     #### Example fine-mapping results ####