Add privileges test for pg_stat_statements to improve coverage

Hi hackers,

I proposal adding privileges test to improve
test coverage of pg_stat_statements.

## test procedure

./configure --enable-coverage --enable-tap-tests --with-llvm CFLAGS=-O0
make check-world
make coverage-html

## coverage

before Line Coverage 74.0 %(702/949 lines)
after Line Coverage 74.4 %(706/949 lines)

Although the improvement is small, I think that test regarding
privileges is necessary.

As a side note,
Initially, I was considering adding a test for dealloc.
However, after reading the thread below, I confirmed that
it is difficult to create tests due to differences due to endian.
(https://www.postgresql.org/message-id/flat/40d1e4f2-835f-448f-a541-8ff5db75bf3d%40eisentraut.org)
For this reason, I first added a test about privileges.

Best Regards,

Keisuke Kuroda
NTT Comware