Replace log.Fatal in tests, add t.Helper, cover NewFile

[dolph]

Summary

Three test-infrastructure fixes from #33. No production code changes.

• log.Fatal* -> t.Fatalf / b.Fatalf in newTestFile, newTestDir, and TestHandleFileWithIgnoredDir. log.Fatal calls os.Exit(1), which kills the test binary, leaks temp files (deferred cleanup doesn't run), and prevents subsequent tests from running. newTestFile / newTestDir now take testing.TB as the first arg so they work for both *testing.T and *testing.B call sites; all call sites are updated.
• Add t.Helper() / b.Helper() as the first line of every test helper that takes *testing.T or *testing.B: assertFileExists, assertFileNonexistent, assertPathExistsAfterRename, assertNewContentsOfFile, assertRandomStringLength, newTestFile, newTestDir, and CloneRepoToTestDir. Failures now point to the call site, not the helper body.
• Replace empty TestNewFile with table-driven coverage of NewFile's path-resolution behavior: absolute path returned cleaned, redundant separators collapsed, .. resolved, relative path resolved to absolute, and "." resolved to the working directory. Uses t.TempDir() and t.Run(tc.name, ...).

Judgment calls

• TestNewFile error path not covered. NewFile currently calls log.Fatalf if filepath.Abs fails, which would kill the test binary. Per scope discipline, exposing this error path belongs to log.Fatal from worker goroutines leaves the tree in a partially-modified state #6's refactor, not this PR. A comment in TestNewFile documents the omission.
• newTestFile / newTestDir signature change to testing.TB. Necessary so CloneRepoToTestDir (a *testing.B helper) can call newTestDir after the latter stops using log.Fatal. All in-tree call sites are updated. No production code is affected.
• Adjacent defects deliberately left alone (would expand scope into log.Fatal from worker goroutines leaves the tree in a partially-modified state #6 or new issues): CloneRepoToTestDir's defer os.Remove(d.Path) runs before the returned dir is used (pre-existing bug); b.Errorf instead of b.Fatalf after a failed git clone; TestHandleFileWithIgnoredDir's use of os.TempDir() instead of t.TempDir().

Verifying t.Helper() correctness

Temporarily added a throwaway test that calls assertFileExists against a nonexistent path. The reported failure line was the call site (zz_helper_sanity_test.go:10), not the helper body (find_replace_test.go:62), confirming t.Helper() works as intended. The throwaway file was removed before commit.

Test plan

• gofmt -l . -- no output
• go vet ./... -- no output
• go build ./... -- no output
• go test -race ./... -- PASS (ok github.com/dolph/find-replace 1.027s)
• ./build.sh -- PASS
• Verified t.Helper() attribution via throwaway test (described above)

Closes #33

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