Make c-ts-mode indent tests side-effect-free

Make c-ts-mode indent tests side-effect-free

Running indent tests changes the global value of
c-ts-mode-indent-style.  That's not good.  This change fixes that.

I also refactored the indent style functions a bit.

* lisp/progmodes/c-ts-mode.el:
(c-ts-mode--prompt-for-style): New function.
(c-ts-mode-set-local-style): New function.
(c-ts-mode-set-style): Use c-ts-mode--prompt-for-style.  Use
derived-mode-p when testing for major mode.  Remove check of current
buffer's major mode since it doesn't matter.

* test/lisp/progmodes/c-ts-mode-resources/indent-bsd.erts:
* test/lisp/progmodes/c-ts-mode-resources/indent.erts: Use
c-ts-mode-set-local-style to set the indent style locally.