Improve coverage of pltcl regression tests.

Tom Lane <tgl@sss.pgh.pa.us>

Commit: 68c521eb92c3515e3306f51a7fd3f32d16c97524
Author: Tom Lane <tgl@sss.pgh.pa.us>
Date: 2016-03-02T01:01:16Z
Releases: 9.6.0
Improve coverage of pltcl regression tests.

Test composite-type arguments and the argisnull and spi_lastoid Tcl
commmands.  This stuff was not covered before, but needs to be exercised
since the upcoming Tcl object-conversion patch changes these code paths
(and broke at least one of them).

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