Add gitignore files for ecpg regression tests.

Backpatch to 8.2 as that's how far the structure looks the same.

src/interfaces/ecpg/test/.gitignore

+/pg_regress
+
+# Exclude all C files in subdirectories
+/*/*.c
+
+# Exclude subdirectories
+/log/
+/results/
+/tmp_check/

src/interfaces/ecpg/test/compat_informix/.gitignore

+# Exclude all non-pgc files
+*
+!*.pgc

src/interfaces/ecpg/test/connect/.gitignore

+# Exclude all non-pgc files
+*
+!*.pgc

src/interfaces/ecpg/test/pgtypeslib/.gitignore

+# Exclude all non-pgc files
+*
+!*.pgc

src/interfaces/ecpg/test/preproc/.gitignore

+# Exclude all non-pgc files
+*
+!*.pgc

src/interfaces/ecpg/test/sql/.gitignore

+# Exclude all non-pgc files
+*
+!*.pgc

src/interfaces/ecpg/test/thread/.gitignore

+# Exclude all non-pgc files
+*
+!*.pgc