[YSQL] Side-by-side diff doesn't pick best expectfile

Jira Link: DB-18417

Description

side-by-side diff logic (which prints a side-by-side diff of regress test expected/actual output into the test log) does not take alternate expectfiles into account. It just takes the non-alternate expectfile, even if it may not give the best diff. This makes reading the side-by-side diff unreliable for use.

PgRegressRunner.java:

      LOG.warn("Failed tests: {}", sortedFailedTests);
      for (String testName : sortedFailedTests) {
        File expectedFile = new File(new File(pgRegressOutputDir, "expected"), testName + ".out");
        File resultFile = new File(new File(pgRegressOutputDir, "results"), testName + ".out");
        if (!expectedFile.exists()) {
          LOG.warn("Expected test output file {} not found.", expectedFile);
          continue;
        }
        if (!resultFile.exists()) {
          LOG.warn("Actual test output file {} not found.", resultFile);
          continue;
        }
        LOG.warn("Side-by-side diff between expected output and actual output of {}:\n{}",
            testName, SideBySideDiff.generate(expectedFile, resultFile));
      }

On the other hand, PG's logic does take it into account.

pg_regress.c:

        if (l < best_line_count)
        {
            /* This diff was a better match than the last one */
            best_line_count = l;
            strlcpy(best_expect_file, alt_expectfile, sizeof(best_expect_file));
        }

If you are hitting this case, looking at the unified diff above the side-by-side diff is recommended.

Discovered this by looking at failures for test ./yb_build.sh --java-test 'org.yb.pgsql.TestPgRegressInsertOnConflict#schedule', which is unfortunately flaky right now.

Issue Type

kind/enhancement

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