add tests for reloading ssh config files

This adds 2 tests. The first checks that ssh config data is cached and
does not update if the file changes afterward. The second checks that
the `ReloadConfigs()` function will discard the cached data and load the
current config file contents.

testdata/modified

@@ -0,0 +1,4 @@
+
+Host wap
+  User modified2
+  KexAlgorithms diffie-hellman-group1-sha1