test: Add a make install target for the tests

This makes it more convenient to compile them alongside SDL, install
them in an optional package and use them as smoke-tests or diagnostic
tools. The default installation directory is taken from GNOME's
installed-tests, which seems as good a convention as any other:
https://wiki.gnome.org/Initiatives/GnomeGoals/InstalledTests

Signed-off-by: Simon McVittie <smcv@collabora.com>

test/Makefile.in

@@ -2,6 +2,15 @@
 
 srcdir  = @srcdir@
 
+prefix = @prefix@
+exec_prefix = @exec_prefix@
+bindir = @bindir@
+libdir = @libdir@
+libexecdir = @libexecdir@
+includedir = @includedir@
+datarootdir = @datarootdir@
+datadir = @datadir@
+
 CC      = @CC@
 EXE	= @EXE@
 CFLAGS  = @CFLAGS@ -g
@@ -81,6 +90,12 @@ TARGETS = \
 
 all: Makefile $(TARGETS) copydatafiles
 
+installedtestsdir = $(libexecdir)/installed-tests/SDL2
+
+install: all
+	install -D -t $(DESTDIR)$(installedtestsdir) $(TARGETS)
+	install -m644 -D -t $(DESTDIR)$(installedtestsdir) $(DATA)
+
 Makefile: $(srcdir)/Makefile.in
 	$(SHELL) config.status $@