Rename tgbatest into tests.

* src/Makefile.am, README, configure.ac: update references.
* src/tgbatest/: rename as...
* src/tests/: ...this!

src/tgbatest/.gitignore

@@ -1,41 +0,0 @@
-acc
-bddprod
-bitvect
-blue_counter
-checkpsl
-checkta
-complement
-defs
-.deps
-*.dot
-eltl2tgba
-emptchk
-expldot
-explicit
-explicit2
-explicit3
-explprod
-input
-intvcomp
-intvcmp2
-.libs
-ltl2tgba
-ltlmagic
-ltlprod
-Makefile
-Makefile.in
-maskacc
-mixprod
-output1
-output2
-powerset
-*.ps
-randtgba
-readsat
-readsave
-reduccmp
-reductgba
-spotlbtt
-taatgba
-tgbaread
-tripprod