ZTS: Simplify zpool_initialize_verify_initialized

Consider the test to be a success as long as the initializing pattern
is found at least once per metaslab. This indicates that at least
part of the free space was initialized. Ideally we'd check that the
pattern was written to all free space but that's much trickier so this
check is a reasonable compromise.

Using a here-string to feed the loop in this test causes an empty
string to still trigger the loop so we miss the spacemaps=0 case.
Pipe into the loop instead.

While here, we can use zpool wait -t initialize $TESTPOOL to wait for
the pool to initialize.

Co-authored-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ryan Moeller <ryan@iXsystems.com>
Closes #11365