Don't build dltest for Tcl 8.x any more #4733

Job	Run time
gcc	1m 41s
msvc	58s
gcc (CFLAGS=-DTCL_NO_DEPRECATED=1)	1m 28s
msvc (CHECKS=nodep)	56s
gcc (--disable-shared)	1m 51s
msvc (OPTS=static)	58s
gcc (--disable-zipfs)	1m 42s
msvc (OPTS=noembed)	1m 1s
gcc (--enable-symbols)	1m 51s
msvc (OPTS=symbols)	59s
gcc (--enable-symbols=mem)	1m 41s
msvc (OPTS=symbols STATS=compdbg,memdbg)	59s
gcc (--enable-symbols=all)	1m 46s
	17m 51s

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