pgsql: Remove memory leaks in isolationtester.

Remove memory leaks in isolationtester.

specscanner.l leaked a kilobyte of memory per token of the spec file.
Apparently somebody thought that the introductory code block would be
executed once; but it's once per yylex() call.

A couple of functions in isolationtester.c leaked small amounts of
memory due to not bothering to free one-time allocations. Might
as well improve these so that valgrind gives this program a clean
bill of health. Also get rid of an ugly static variable.

Coverity complained about one of the one-time leaks, which led me
to try valgrind'ing isolationtester, which led to discovery of the
larger leak.

Branch
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REL_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/bc031cf133b786ab3c3def9d28b044b85a743edc

Modified Files
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src/test/isolation/isolationtester.c | 20 +++++++++++++-------
src/test/isolation/specscanner.l | 8 ++++++--
2 files changed, 19 insertions(+), 9 deletions(-)