Fix all clippy warnings across the codebase

[jgarzik]

• awk: Change test values to avoid approx_constant and excessive_precision
• tree/tests: Fix option_env_unwrap by using let-else pattern
• m4/tests: Remove unnecessary & before Path::new calls
• sh/wordexp: Use struct init syntax instead of field reassignment
• editors/tests: Use struct init syntax instead of field reassignment
• sccs/tests: Change &PathBuf to &Path in function signatures
• tree/tests/chgrp: Rename _1/_2/_3 to dir1/dir2/dir3
• tree/tests/chmod: Change filter_map to filter
• process/tests/fuser: Add .wait() after .kill() for zombie processes
• file/tests/cmp: Use iterator instead of index loop
• Various auto-fixes via clippy --fix: useless_format, let_and_return, expect_fun_call, needless_borrow, let_unit_value, redundant pattern matching, to_string_in_format_args, trim before split_whitespace

[Copilot]

Pull request overview

This PR addresses clippy warnings across the entire posixutils-rs codebase by applying various automatic and manual fixes to improve code quality and consistency.

Changes:

• Removed unnecessary type conversions, format macros, and borrows
• Changed test values to avoid clippy approx_constant and excessive_precision warnings
• Updated test patterns to use let-else instead of expect for optional environment variables
• Changed function parameters from &PathBuf to &Path for better ergonomics
• Improved variable naming in test code
• Added .wait() calls after .kill() to prevent zombie processes
• Fixed iterator usage patterns

Reviewed changes

Copilot reviewed 31 out of 31 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
users/tests/write/mod.rs	Simplified arithmetic by removing unnecessary 0u8 +
tree/tests/rm/mod.rs	Changed option_env handling from expect to let-else pattern
tree/tests/readlink/mod.rs	Replaced redundant format! with direct to_string()
tree/tests/mv/mod.rs	Removed unnecessary to_string() in format!, updated option_env handling
tree/tests/ls/mod.rs	Removed redundant field name, simplified conditionals, removed unnecessary format!
tree/tests/cp/mod.rs	Updated option_env handling to let-else pattern
tree/tests/chown/mod.rs	Changed iterator from into_iter to iter, removed unnecessary format! calls
tree/tests/chmod/mod.rs	Changed filter_map to filter for simpler boolean logic
tree/tests/chgrp/mod.rs	Renamed variables from _1/_2/_3 to dir1/dir2/dir3 for clarity
text/tests/csplit/mod.rs	Changed vec! to array literal for fixed-size collections
sys/tests/uname/mod.rs	Removed redundant trim before split_whitespace
sh/wordexp/pathname.rs	Changed into_iter to iter for borrowed iteration
sh/wordexp/parameter.rs	Used struct initialization syntax instead of field reassignment
sh/tests/integration.rs	Removed unnecessary reference, added spin_loop hint
sh/pattern/regex.rs	Removed redundant reference
sccs/tests/delta/mod.rs	Changed &PathBuf to &Path in function signatures
process/tests/timeout/mod.rs	Changed expect with format to unwrap_or_else, simplified iteration
process/tests/fuser/with_user.rs	Added wait() after kill() to prevent zombie processes
process/tests/fuser/basic.rs	Added wait() after kill() to prevent zombie processes
plib/src/sccsfile.rs	Changed expect with format to unwrap_or_else
man/man_util/formatter.rs	Replaced redundant format! with direct to_string()
m4/tests/integration_test.rs	Added unnecessary references to Path::new calls
m4/test-manager/src/lib.rs	Changed assert_eq with true to assert!
ftw/tests/integration.rs	Replaced redundant format! with to_string()
file/tests/file/mod.rs	Changed if let Ok(_) pattern to is_ok(), removed let _ = pattern
file/tests/cmp/mod.rs	Changed indexed loop to iterator, changed char as u8 to byte literal
editors/tests/integration/mod.rs	Used struct initialization syntax instead of field reassignment
datetime/tests/time/mod.rs	Removed let-and-return pattern
cron/tests/crontab/mod.rs	Changed expect with format to unwrap_or_else, removed let-and-return
cron/tests/crond/mod.rs	Changed expect with format to unwrap_or_else, removed let-and-return, removed let _ = pattern
awk/interpreter/format.rs	Changed test values to avoid excessive_precision warnings