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.}

tests/assert/t21195.nim

@@ -0,0 +1,6 @@
+discard """
+  matrix: "--verbosity:0 --os:standalone --mm:none"
+"""
+# bug #21195
+var n = 11
+assert(n == 12)

tests/assert/tassert_c.nim

@@ -1,8 +1,8 @@
 discard """
-  cmd: "nim $target $options --excessiveStackTrace:off $file"
+  cmd: "nim $target $options -d:nimPreviewSlimSystem --excessiveStackTrace:off $file"
   output: '''true'''
 """
-
+import std/assertions
 const expected = """
 tassert_c.nim(35)        tassert_c
 tassert_c.nim(34)        foo