Run repack_dropped with make and clean up its extension

Commit e5d25959cf added the repack_dropped test to Meson but omitted it
from the Makefile, so make-based isolation runs did not execute it. The
test's teardown also left the injection_points extension installed,
relying on a later test to remove it.

Add repack_dropped to the Makefile's isolation tests and drop the
extension during teardown.

Author: Fujii Masao <masao.fujii@gmail.com>
Reviewed-by: Álvaro Herrera <alvherre@kurilemu.de>
Discussion: https://postgr.es/m/CAHGQGwG9bHxhs4Nj+9avSjSGh0vdYNQgEuz+pmJ-H5be6tEBTw@mail.gmail.com
Backpatch-through: 19

src/test/modules/injection_points/Makefile

@@ -25,6 +25,7 @@ ISOLATION = basic \
 	    repack \
 	    repack_commit_race \
 	    repack_decode \
+	    repack_dropped \
 	    repack_missingval \
 	    repack_temporal \
 	    repack_temporal_multirange \

src/test/modules/injection_points/specs/repack_dropped.spec

@@ -24,6 +24,7 @@ setup {
 teardown {
 	DROP TABLE repack_dropped;
 	DROP FUNCTION repack_dropped_f;
+	DROP EXTENSION injection_points;
 }
 
 session s1