pgsql: Make sure list of tests is cleared out before getting the new

Log Message:
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Make sure list of tests is cleared out before getting the new list
of tests. Per Andrew Dunstan.

Modified Files:
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pgsql/src/tools/msvc:
vcregress.bat (r1.7 -> r1.8)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/tools/msvc/vcregress.bat.diff?r1=1.7&r2=1.8)