test: skip_fragile(), TEST_SKIP_FRAGILE

Let build systems define TEST_SKIP_FRAGILE to skip tests that are known to be
resource-intensive (unreliable on slow systems).

References https://github.com/neovim/neovim/pull/5488#issuecomment-265622113

test/functional/ex_cmds/ctrl_c_spec.lua

@@ -11,6 +11,12 @@ describe("CTRL-C (mapped)", function()
   end)
 
   it("interrupts :global", function()
+    if helpers.skip_fragile(pending,
+      (os.getenv("TRAVIS") and os.getenv("CLANG_SANITIZER") == "ASAN_UBSAN"))
+    then
+      return
+    end
+
     source([[
       set nomore nohlsearch undolevels=-1
       nnoremap <C-C> <NOP>