fix #17853 (ascii message separator broke json nim dump) (#17887)

tests/misc/trunner.nim

@@ -71,6 +71,7 @@ ret=[s1:foobar s2:foobar age:25 pi:3.14]
 
 else: # don't run twice the same test
   import std/strutils
+  import std/json
   template check2(msg) = doAssert msg in output, output
 
   block: # tests with various options `nim doc --project --index --docroot`
@@ -317,3 +318,13 @@ compiling: v3
 running: v3
 running: v2
 """, ret
+
+  block: # nim dump
+    let cmd = fmt"{nim} dump --dump.format:json -d:D20210428T161003 --hints:off ."
+    let (ret, status) = execCmdEx(cmd)
+    doAssert status == 0
+    let j = ret.parseJson
+    # sanity checks
+    doAssert "D20210428T161003" in j["defined_symbols"].to(seq[string])
+    doAssert j["version"].to(string) == NimVersion
+    doAssert j["nimExe"].to(string) == getCurrentCompilerExe()