disable macOS tests

our macOS tests fails more than half the time while linux and windows
pass. the errors are always due to timeout issues.

while we can improve the performance of our tests (particularly
test/messages.ts) sometimes the timeouts happen when testing against the
local SMTPServer

as of now, we can't get insight into whether our tests or passing or
failing as the majority of test fails when macOS is included.

github is away of the issue: actions/runner-images#3885

.github/workflows/test.yml

@@ -8,7 +8,7 @@ jobs:
       fail-fast: false
       matrix:
         node: [^12, ^14, ^16, ^18]
-        os: [ubuntu-latest, windows-latest, macos-latest]
+        os: [ubuntu-latest, windows-latest]
 
     steps:
       - name: checkout