[LIT] Migrate tests lit config to the internal shell (#24810)

Part of #24607, following #24794.

This moves `tests/lit.cfg.py` to the internal shell and drops the
`execute_external` / `force_execute_external` workaround. No RUN lines
needed changes this time. None of the 27 lit tests under `tests/` use
subshells, or anything else the internal shell rejects.

All 27 pass, same as before the change. To check that they still
actually run and aren't just being skipped, I broke a CHECK pattern in
`e2e/regression/globals.mlir` on purpose, confirmed the test failed,
then put it back.

Verified on macOS/arm64 only.

Signed-off-by: Zohaib Shah <zohaib.shah.2047@gmail.com>
Co-authored-by: Zohaib Shah <zohaib.shah.2047@gmail.com>

tests/lit.cfg.py

diff --git a/tests/lit.cfg.py b/tests/lit.cfg.py
index 81bc517..e0c9133 100644
--- a/tests/lit.cfg.py
+++ b/tests/lit.cfg.py
@@ -17,9 +17,7 @@
 
 config.name = "IREE"
 config.suffixes = [".mlir", ".txt"]
-config.test_format = lit.formats.ShTest(
-    execute_external=True, force_execute_external=True
-)
+config.test_format = lit.formats.ShTest()
 
 # Forward all IREE environment variables, as well as some passthroughs.
 # Note: env vars are case-insensitive on Windows, so check matches carefully.