Have test_vfs.c simulate IO errors in xShmLock.

FossilOrigin-Name: fcbf7cf189506e43fc2f0820aedffb195038d3a9

test/wal_common.tcl

@@ -0,0 +1,26 @@
+# 2010 June 03
+#
+# The author disclaims copyright to this source code.  In place of
+# a legal notice, here is a blessing:
+#
+#    May you do good and not evil.
+#    May you find forgiveness for yourself and forgive others.
+#    May you share freely, never taking more than you give.
+#
+#***********************************************************************
+#
+# This file contains common code used by many different malloc tests
+# within the test suite.
+#
+
+proc wal_file_size {nFrame pgsz} {
+  expr {24 + ($pgsz+24)*$nFrame}
+}
+
+proc wal_frame_count {zFile pgsz} {
+  set f [file size $zFile]
+  expr {($f - 24) / ($pgsz+24)}
+}
+
+
+