[LU-20255] Add new fsx test variations

• Type: Improvement
• Resolution: Not a Bug
• Priority: Medium
• Labels: TC-triage-followup

Epic/Theme

test

• Severity: 3
• Assignee: WC Triage
• Reporter: Christopher Davis

Description

Add four new fsx test variations to `sanity-benchmark.sh` to expand functional coverage for custom large operations using 64KB block sizes.
 
The new tests verify Lustre's behavior under heavy I/O stress with the following configurations:

• `test_fsx_custom_direct`: Large block ops (64KB) using O_DIRECT.
• `test_fsx_custom_buffered`: Large block ops (64KB) using buffered I/O.
• `test_fsx_custom_no_mmap`: Large block ops (64KB) with O_DIRECT and mmap disabled.
• `test_fsx_custom_default_layout`: Large block ops (64KB) with O_DIRECT using the default striping layout instead of forcing `-c -1`.
   
  *Key Behaviors:*
• All tests utilize the `-Z` flag to bypass standard randomization in `fsx.c`, guaranteeing the intended I/O mode is exercised.
• Each test includes dynamic size scaling to automatically reduce the test file size if available mount space is limited, preventing false failures.