This is helpful for separating user test cases from each other, and also
fixes a bug wherby `UTEST_COLOUR_OUTPUT` implicity sets `errno` if the
standard streams are not connected to a tty. e.g.

```shell-session
$ cat <<EOF > test.c

UTEST(errno, not_set)
{
    ASSERT_EQ(errno, 0);
}

UTEST_MAIN();
EOF

$ cc test.c
$ ./a.out
[==========] Running 1 test cases.
[ RUN      ] errno.not_set
[       OK ] errno.not_set (1747ns)
[==========] 1 test cases ran.
[  PASSED  ] 1 tests.
$ ./a.out | cat
[==========] Running 1 test cases.
[ RUN      ] errno.not_set
test.c:6: Failure
  Expected : 25
    Actual : 0
[  FAILED  ] errno.not_set (4892ns)
[==========] 1 test cases ran.
[  PASSED  ] 0 tests.
[  FAILED  ] 1 tests, listed below:
[  FAILED  ] errno.not_set
```