Fix extended tests by restore datafusion-testing submodule

[alamb]

Which issue does this PR close?

Rationale for this change

Extended tests are failing on main. Example: https://github.com/apache/datafusion/actions/runs/13955086498/job/39064031622

    Finished `release-nonlto` profile [optimized] target(s) in 11m 58s
     Running bin/sqllogictests.rs (target/release-nonlto/deps/sqllogictests-e83c87031ca01d50)
Error: Execution("Error reading directory \"../../datafusion-testing/data/\": No such file or directory (os error 2)\n\nbacktrace:    0: <unknown>\n   1: <unknown>\n   2: <unknown>\n   3: <unknown>\n   4: <unknown>\n   5: <unknown>\n   6: <unknown>\n   7: <unknown>\n   8: <unknown>\n   9: <unknown>\n  10: <unknown>\n  11: <unknown>\n  12: __libc_start_main\n  13: <unknown>\n")
error: test failed, to rerun pass `-p datafusion-sqllogictest --test sqllogictests`

Caused by:
  process didn't exit successfully: `/__w/datafusion/datafusion/target/release-nonlto/deps/sqllogictests-e83c87031ca01d50 --include-sqlite` (exit status: 1)

This is because we accidentally removed the datafusion-testing submodule in #15263

What changes are included in this PR?

Restore the pin

Are these changes tested?

Are there any user-facing changes?

[alamb]

Thanks @adriangb and @ozankabak