assertions: fixes #11545 (#11605)

* assertions: properly fix #11545

* tests/assert: enable excessiveStackTrace

* tests/assert: add test case for #11545

* tfailedassert_stacktrace: disable excessiveStackTrace

* assertions: weird workaround for failing tests

This fixes megatest on *nix, but have no idea why

tests/assert/trelativeassert.nim

@@ -0,0 +1,11 @@
+discard """
+  cmd: "nim $target $options --excessiveStackTrace:off $file"
+  output: '''
+test:ok
+'''
+"""
+import testhelper
+try:
+  doAssert(false, "msg")
+except AssertionError as e:
+  checkMsg(e.msg, "trelativeassert.nim(9, 11) `false` msg", "test", false)