Fixture for tools/mutation-table.sh. Expected: add-minus CAUGHT by add_adds,
even-flip CAUGHT by even_detects, clamp-positive SURVIVED (no test reads the
positive branch), add-typo UNVIABLE (with BUILD='cargo test --no-run'),
ghost NOAPPLY. Run with TEST='cargo test' BUILD='cargo test --no-run'.

=== add-minus
file: src/lib.rs
--- before
    a + b
--- after
    a - b

=== even-flip
file: src/lib.rs
test: cargo test even_
--- before
n % 2 == 0
--- after
n % 2 == 1

=== clamp-positive
file: src/lib.rs
--- before
else { n }
--- after
else { n + 1 }

=== add-typo
file: src/lib.rs
--- before
    a + b
--- after
    a + c

=== ghost
file: src/lib.rs
--- before
this text is nowhere
--- after
x
