tests: Test metadata checksum recalculation a bit

Change metadata checksum recalculation after all kind of metadata
changes, but without any ongoing operations during the
recalculation itself.

Change-Id: I4459d20f2ba97ed6e8b497dfa9df40e892418d5d
tests: Test metadata checksum recalculation under filesystem load

This test verifies if the master server is able to correctly recalculate
checksum of its metadata if the filesystem is constantly changing.

Change-Id: Ifa3cd3ba954ba421a9d83939946de21ef17cb664
tests: Fix test_shadow_checksum_error_recovery

Make test_shadow_checksum_error_recovery not fail when debug log is used
for other purposes than needed here.

Change-Id: I1599ac4c3e4fe9a38d500da5a524aed397160bd8