CI openbsd: 3x batching via NIM_TESTAMENT_BATCH ; overall CI finishes in 21m instead of 34m (#14851)

* CI openbsd: 2x batching via NIM_TESTAMENT_BATCH

* auto-generate .builds/openbsd_x.yml to avoid code duplication

* 3x batching

tools/ci_generate.nim

@@ -0,0 +1,63 @@
+##[
+avoid code duplication in CI pipelines.
+For now, this is only used for openbsd, but there is a lot of other code
+duplication that could be removed.
+
+## usage
+edit this file as needed and then re-generate via:
+```
+nim r tools/ci_generate.nim
+```
+]##
+
+import std/strformat
+
+proc genCIopenbsd(batch: int, num: int): string =
+  result = fmt"""
+## do not edit directly; auto-generated by `nim r tools/ci_generate.nim`
+
+image: openbsd/latest
+packages:
+- gmake
+- sqlite3
+- node
+- boehm-gc
+- pcre
+- sfml
+- sdl2
+- libffi
+sources:
+- https://github.com/nim-lang/Nim
+environment:
+  NIM_TESTAMENT_BATCH: "{batch}_{num}"
+  CC: /usr/bin/clang
+tasks:
+- setup: |
+    cd Nim
+    git clone --depth 1 -q https://github.com/nim-lang/csources.git
+    gmake -C csources -j $(sysctl -n hw.ncpuonline)
+    bin/nim c koch
+    echo 'export PATH=$HOME/Nim/bin:$PATH' >> $HOME/.buildenv
+- test: |
+    cd Nim
+    if ! ./koch runCI; then
+      nim c -r tools/ci_testresults.nim
+      exit 1
+    fi
+triggers:
+- action: email
+  condition: failure
+  to: Andreas Rumpf <rumpf_a@web.de>
+"""
+
+proc main()=
+  let num = 3
+    # if you reduce this, make sure to remove files that shouldn't be generated,
+    # or better, do the cleanup logic here eg: `rm .builds/openbsd_*`
+  for i in 0..<num:
+    let file = fmt".builds/openbsd_{i}.yml"
+    let code = genCIopenbsd(i, num)
+    writeFile(file, code)
+
+when isMainModule:
+  main()