test: remove BUILT_SOURCES; add missing dependencies.

test: remove BUILT_SOURCES; add missing dependencies.

Without BUILT_SOURCES in Makefile, no test sources in tests/ will be
built when using the default "make all" target. This is desireable for
people who are just building flex but who do not want to run, and
therefore, build any part of the test suite.

Also fix missing sources dependencies:

    bison_nr_scanner.$(OBJEXT): bison_nr_parser.h
    bison_yylloc_scanner.$(OBJEXT): bison_yylloc_parser.h
    bison_yylval_scanner.$(OBJEXT): bison_yylval_parser.h