oeqa/selftest/context.py: For -t/-T use append argparse action

oeqa/selftest/context.py: For -t/-T use append argparse action

Use the 'append' action of argparse instead of nargs. This changes the
behaviour of the option from "-t foo bar -r" to "-t foo -t bar -r".

Additionally rename the long form options to be consistent with
behaviour, such that they specifying a single tag at a time.

Signed-off-by: Nathan Rossi <nathan@nathanrossi.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>