RUST_TEST_TASKS=1 colors output when stdout is not a tty

[bnoordhuis]

Apologies if this has been reported before. It's a minor issue for sure but here goes:

$ echo ' #[test] fn test() {}' | rustc --test -o t -

$ env RUST_TEST_TASKS=1 ./t | xxd  # contains escape codes
0000000: 0a72 756e 6e69 6e67 2031 2074 6573 740a  .running 1 test.
0000010: 7465 7374 2074 6573 7420 2e2e 2e20 1b5b  test test ... .[
0000020: 3332 6d6f 6b1b 5b6d 0f0a 0a74 6573 7420  32mok.[m...test 
0000030: 7265 7375 6c74 3a20 1b5b 3332 6d6f 6b1b  result: .[32mok.
0000040: 5b6d 0f2e 2031 2070 6173 7365 643b 2030  [m.. 1 passed; 0
0000050: 2066 6169 6c65 643b 2030 2069 676e 6f72   failed; 0 ignor
0000060: 6564 3b20 3020 6d65 6173 7572 6564 0a0a  ed; 0 measured..

$ env RUST_TEST_TASKS=2 ./t | xxd  # clean
0000000: 0a72 756e 6e69 6e67 2031 2074 6573 740a  .running 1 test.
0000010: 7465 7374 2074 6573 7420 2e2e 2e20 6f6b  test test ... ok
0000020: 0a0a 7465 7374 2072 6573 756c 743a 206f  ..test result: o
0000030: 6b2e 2031 2070 6173 7365 643b 2030 2066  k. 1 passed; 0 f
0000040: 6169 6c65 643b 2030 2069 676e 6f72 6564  ailed; 0 ignored
0000050: 3b20 3020 6d65 6173 7572 6564 0a0a       ; 0 measured..