du: tests: fix the H_flag test (primarily grep usage)

This test attempts to use \t (tab intended) in a grep expression. With the
former /usr/bin/grep (i.e. gnugrep), this was interpreted as a literal 't'.
The expression would work anyways because the tr(1) usage would ultimately
replace all of the spaces with a single newline, and they would match the
paths whether they were correctly fromatted or not.

Current /usr/bin/grep (i.e. bsdgrep) is less-tolerant of ordinary-escapes, a
property of the underlying regex(3) engine, to make it easier to identify
when stuff like this happens. In-fact, this expression broke after the
switch happened.

This revision does the bare basics to fix the usage by using a printf to get
a literal tab character to insert into the expression. It also swaps out the
manual insertion of the line prefix into the grep expression by pulling
that part out of $sep and reusing it for the leading path.

The secondary issue was the tr(1) usage, since tr would only replace the
first character of string1 with the first character of string2. This has
instead been replaced by a sed expression, which similary understands \n to
be a newline on all supported versions of FreeBSD. Each path now gets
prefixed with the appropriate context that should be there (i.e. numeric
sequence followed by a tab).

PR: 252446
Reviewed by: emaste, ngie
Differential Revision: https://reviews.freebsd.org/D27983