Avoid accidental wildcard expansion in msys shell

Avoid accidental wildcard expansion in msys shell

Commit f092de05 added a test for pg_dumpall --exclude-database including
the wildcard pattern '*dump*' which matches some files in the source
directory. The test library on msys uses the shell which expands this
and thus the program gets incorrect arguments. This doesn't happen if
the pattern doesn't match any files, so here the pattern is set to
'*dump_test*' which is such a pattern.

Per buildfarm animal jacana.