test: actually report the failure in the log

[gadfort]

Summary

Print the reason of the test failure, the diff is not particularly clear when the failure is a missing newline.

Type of Change

• New feature

Impact

None, just improve feedback on tests.

Verification

• I have verified that the local build succeeds (./etc/Build.sh).
• I have run the relevant tests and they pass.
• My code follows the repository's formatting guidelines.
• I have signed my commits (DCO).

[gemini-code-assist]

Code Review

This pull request standardizes indentation to spaces in test/regression_test.sh and improves error handling by ensuring the script exits with a non-zero status and prints an error message when log diffs or pass/fail checks fail. The review feedback suggests enhancing debuggability in CI environments by printing the actual diff (using diff -u) and the last few lines of the log file directly to standard output upon failure.