Fix backtrace sed script for non-GNU seds

Tested on GNU `sed --posix`.

From `info sed`:
`\+'
     As `*', but matches one or more.  It is a GNU extension.

`\CHAR'
     Matches CHAR, where CHAR is one of `$', `*', `.', `[', `\', or `^'.
     Note that the only C-like backslash sequences that you can
     portably assume to be interpreted are `\n' and `\\'; in particular
     `\t' is not portable, and matches a `t' under most implementations
     of `sed', rather than a tab character.

libexec/bats-exec-test

@@ -153,7 +153,7 @@ bats_print_stack_trace() {
       # inspired by Gentoo Portage die()
       failed_cmd="$(sed \
         -e "${failed_cmd_line}!d" \
-        -e "${failed_cmd_line}s:^[ \t]\+::" \
+        -e "${failed_cmd_line}s:^[ 	][ 	]*::" \
         -e "${failed_cmd_line}q" \
         "${failed_cmd_filename}")"
       echo "#   \`${failed_cmd}' failed"