Fix bug in parsing TEL content. Rework unit tests.

- newVC3_Tel was not assigning the value provided to the constructed
  object.
- Private unit tests were run every time the code was compiled due to
  how the unittest library works. These now only run as part of the unit
  tests with `nimble test`.

tests/tlexer.nim

@@ -1,2 +1,6 @@
 import unittest
-import vcard/private/lexer
+import ./vcard/private/lexer
+
+suite "vcard/private/lexer":
+  test "private lexer tests":
+    runVcardLexerPrivateTests()