fixes #21195; std/assertions continue to use sysFatal when nimPreviewSlimSystem is not defined (#21196)

* fixes #21195; `std/assertions` continue to use `sysFatal`

* try includes

* make `std/assertions` self-contained

* fixes tests

* fixes tests

tests/assert/panicoverride.nim

@@ -0,0 +1,15 @@
+# panicoverride.nim
+
+proc printf(fmt: cstring) {.varargs, importc, header:"stdio.h".}
+proc exit(code: cint) {.importc, header:"stdlib.h".}
+
+{.push stack_trace: off, profiler:off.}
+
+proc rawoutput(s: cstring) =
+  printf("RAW: %s\n", s)
+  
+proc panic(s: cstring) {.noreturn.} =
+  printf("PANIC: %s\n", s)
+  exit(0)
+
+{.pop.}