patch 8.0.1690: not easy to run one test with gvim instead of vim

Problem:    Not easy to run one test with gvim instead of vim.
Solution:   Add VIMTESTTARGET in Makefile.

src/Makefile

@@ -2088,6 +2088,10 @@ benchmark:
unittesttargets:
	$(MAKE) -f Makefile $(UNITTEST_TARGETS)

# Swap these lines to run individual tests with gvim instead of vim.
VIMTESTTARGET = $(VIMTARGET)
# VIMTESTTARGET = $(GVIMTARGET)

# Execute the unittests one by one.
unittest unittests: $(RUN_UNITTESTS)

@@ -2115,7 +2119,7 @@ test1 \
	test70 test72 \
	test85 test86 test87 test88 \
	test94 test95 test99 test108:
	cd testdir; rm -f $@.out; $(MAKE) -f Makefile $@.out VIMPROG=../$(VIMTARGET) $(GUI_TESTARG) SCRIPTSOURCE=../$(SCRIPTSOURCE)
	cd testdir; rm -f $@.out; $(MAKE) -f Makefile $@.out VIMPROG=../$(VIMTESTTARGET) $(GUI_TESTARG) SCRIPTSOURCE=../$(SCRIPTSOURCE)

# Run individual NEW style test.
# These do not depend on the executable, compile it when needed.
@@ -2312,14 +2316,14 @@ test_arglist \
	test_alot_latin \
	test_alot_utf8 \
	test_alot:
	cd testdir; rm -f $@.res test.log messages; $(MAKE) -f Makefile $@.res VIMPROG=../$(VIMTARGET) $(GUI_TESTARG) SCRIPTSOURCE=../$(SCRIPTSOURCE)
	cd testdir; rm -f $@.res test.log messages; $(MAKE) -f Makefile $@.res VIMPROG=../$(VIMTESTTARGET) $(GUI_TESTARG) SCRIPTSOURCE=../$(SCRIPTSOURCE)
	@if test -f testdir/test.log; then \
		cat testdir/test.log; \
	fi
	cat testdir/messages

newtests:
	cd testdir; rm -f $@.res test.log messages; $(MAKE) -f Makefile newtestssilent VIMPROG=../$(VIMTARGET) $(GUI_TESTARG) SCRIPTSOURCE=../$(SCRIPTSOURCE)
	cd testdir; rm -f $@.res test.log messages; $(MAKE) -f Makefile newtestssilent VIMPROG=../$(VIMTESTTARGET) $(GUI_TESTARG) SCRIPTSOURCE=../$(SCRIPTSOURCE)
	@if test -f testdir/test.log; then \
		cat testdir/test.log; \
	fi

src/version.c

@@ -762,6 +762,8 @@ static char *(features[]) =

static int included_patches[] =
{   /* Add new patch number below this line */
/**/
    1690,
/**/
    1689,
/**/