utils.io: Improve handling of double and single slashes

Up until now “IsBelowDir("/", …)” would never return True. The reason
was that an additional slash was added to the root path resulting in
“//", which is “implementation-defined” in posix and treated specially
by “os.path.normpath”.

This patch fixes the behaviour for this special case and adds tests
(also for IsNormAbsPath). A typo in the docstring is fixed. Calls to
“assert_” and “assertFalse” are changed to pass a message by keyword
argument.

It is a bit of a mess, but I hope the resulting behaviour is correct.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Dato Simó <dato@google.com>