The original coding of this test used table and view names like "t",
"tv", "foo", etc.  This tended to interfere with doing simple manual
tests in the regression database; not to mention that it posed a
considerable risk of conflict with other regression test scripts.
Prefix these names with "mvtest_" to avoid such conflicts.

Also, change transiently-created role name to be "regress_xxx" per
discussions about being careful with regression-test role creation.