tests: add a test for string's grow behaviour

Author: zerbina

tests/stdlib/strings/tstring_grow_zeroes_memory.nim

@@ -0,0 +1,24 @@
+discard """
+  targets: c js vm
+  knownIssue.js: "The new characters are not set to NUL"
+"""
+
+var a: string
+a.setLen(3)
+doAssert a == "\0\0\0"
+
+# start from a constant string
+var b = "abcdef"
+b.setLen(3)
+doAssert b == "abc"
+b.setLen(6) # grow to previous size
+doAssert b == "abc\0\0\0"
+
+# start from a manually constructed string
+var c: string
+for _ in 0..<4:
+  c.add 'c'
+c.setLen(2)
+doAssert c == "cc"
+c.setLen(4) # grow to previous size
+doAssert c == "cc\0\0"