ZTS: fix removal_condense_export test case

It's been observed in the CI that the required 25% of obsolete bytes
in the mapping can be to high a threshold for this test resulting in
condensing never being triggered and a test failure.  To prevent these
failures make the existing zfs_condense_indirect_obsolete_pct tuning
available so the obsolete percentage can be reduced from 25% to 5%
during this test.

Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #11869

man/man5/zfs-module-parameters.5

@@ -1445,6 +1445,19 @@ to throttle vdev removal speed.
 Default value: \fB0\fR (no throttle).
 .RE
 
+.sp
+.ne 2
+.na
+\fBzfs_condense_indirect_obsolete_pct\fR (int)
+.ad
+.RS 12n
+Minimum percent of obsolete bytes in vdev mapping required to attempt to
+condense (see \fBzfs_condense_indirect_vdevs_enable\fR). Intended for use
+with the test suite to facilitate triggering condensing as needed.
+.sp
+Default value: \fB25\fR%.
+.RE
+
 .sp
 .ne 2
 .na