improve std/tempfiles (#17920)

* improve std/tempfiles
* fixup
* fix windows
* improve test
* improve runnableExamples and tests
* address comment

tests/stdlib/ttempfiles.nim

@@ -1,17 +1,49 @@
-import std/[os, tempfiles]
+discard """
+  joinable: false # not strictly necessary
+"""
 
+import std/tempfiles
+import std/[os, nre]
 
-doAssert createTempDir("nim", "tmp") != createTempDir("nim", "tmp")
+const
+  prefix = "D20210502T100442" # safety precaution to only affect files/dirs with this prefix
+  suffix = ".tmp"
 
 block:
-  var t1 = createTempFile("nim", ".tmp")
-  var t2 = createTempFile("nim", ".tmp")
+  var t1 = createTempFile(prefix, suffix)
+  var t2 = createTempFile(prefix, suffix)
+  defer:
+    close(t1.cfile)
+    close(t2.cfile)
+    removeFile(t1.path)
+    removeFile(t2.path)
+
   doAssert t1.path != t2.path
 
-  write(t1.fd, "1234")
-  doAssert readAll(t2.fd) == ""
+  let s = "1234"
+  write(t1.cfile, s)
+  doAssert readAll(t2.cfile) == ""
+  doAssert readAll(t1.cfile) == ""
+  t1.cfile.setFilePos 0
+  doAssert readAll(t1.cfile) == s
 
-  close(t1.fd)
-  close(t2.fd)
-  removeFile(t1.path)
-  removeFile(t2.path)
+block: # createTempDir
+  doAssertRaises(OSError): discard createTempDir(prefix, suffix, "nonexistent")
+
+  block:
+    let dir1 = createTempDir(prefix, suffix)
+    let dir2 = createTempDir(prefix, suffix)
+    defer:
+      removeDir(dir1)
+      removeDir(dir2)
+    doAssert dir1 != dir2
+
+    doAssert dirExists(dir1)
+    doAssert dir1.lastPathPart.contains(re"^D20210502T100442(\w+).tmp$")
+    doAssert dir1.parentDir == getTempDir()
+
+  block:
+    let dir3 = createTempDir(prefix, "_mytmp", ".")
+    doAssert dir3.lastPathPart.contains(re"^D20210502T100442(\w+)_mytmp$")
+    doAssert dir3.parentDir == "." # not getCurrentDir(): we honor the absolute/relative state of input `dir`
+    removeDir(dir3)