Write backtrace fmt test using relative paths
Write backtrace fmt test using relative paths

For some reason the absolute paths were formatted differently on the
armhf-gnu target.

    thread '<unnamed>' panicked at 'assertion failed: `(left == right)`
      left: `"Backtrace [\n    { fn: \"__rust_maybe_catch_panic\" },\n    { fn: \"std::rt::lang_start_internal\", file: \"./rust/rt.rs\", line: 300 },\n    { fn: \"std::rt::lang_start\", file: \"./rust/rt.rs\", line: 400 },\n]"`,
     right: `"Backtrace [\n    { fn: \"__rust_maybe_catch_panic\" },\n    { fn: \"std::rt::lang_start_internal\", file: \"/rust/rt.rs\", line: 300 },\n    { fn: \"std::rt::lang_start\", file: \"/rust/rt.rs\", line: 400 },\n]"`', src/libstd/backtrace.rs:486:5

graydon/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust