meson: Add pg_regress_ecpg to ecpg test dependencies

This is required to ensure correct test dependencies, previously
pg_regress_ecpg would not necessarily be built.

The missing dependency was, e.g., visible when doing
  ninja clean && ninja meson-test-prereq && meson test --no-rebuild --suite setup --suite ecpg

Apply this to all branches with meson support, as part of an effort to fix
incorrect test dependencies that can lead to test failures.

Discussion: https://postgr.es/m/CAGECzQSvM3iSDmjF+=Kof5an6jN8UbkP_4cKKT9w6GZavmb5yQ@mail.gmail.com
Discussion: https://postgr.es/m/bdba588f-69a9-4f3e-9b95-62d07210a32e@eisentraut.org
Backpatch: 16-, where meson support was added

Author: anarazel

src/interfaces/ecpg/test/meson.build

@@ -5,6 +5,8 @@ if meson.is_cross_build()
   subdir_done()
 endif
 
+ecpg_test_dependencies = []
+
 pg_regress_ecpg_sources = pg_regress_c + files(
   'pg_regress_ecpg.c',
 )
@@ -23,7 +25,7 @@ pg_regress_ecpg = executable('pg_regress_ecpg',
     'install': false
   },
 )
-testprep_targets += pg_regress_ecpg
+ecpg_test_dependencies += pg_regress_ecpg
 
 # create .c files and executables from .pgc files
 ecpg_test_exec_kw = {
@@ -51,8 +53,6 @@ ecpg_preproc_test_command_end = [
   '-o', '@OUTPUT@', '@INPUT@'
 ]
 
-ecpg_test_dependencies = []
-
 subdir('compat_informix')
 subdir('compat_oracle')
 subdir('connect')