revert: removed all "USE"s in sqlness tests introduced in #922

[MichaelScofield]

I hereby agree to the terms of the GreptimeDB CLA

What's changed and what's your intention?

Because the "USE"s in sqlness test cases could introduce another burden for writing tests, it's better to remove them. Thus all test cases are using the default "public" schema if not explicitly specified using "USE". And to test "USE", we have to reset the schema to default "public" using "USE public" at the end of test case.

Checklist

• I have written the necessary rustdoc comments.
• I have added the necessary unit tests and integration tests.

Refer to a related PR or issue link (optional)

Revert part of #922

[codecov]

Codecov Report

Merging #938 (dc795f4) into develop (af93567) will decrease coverage by 0.11%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop     #938      +/-   ##
===========================================
- Coverage    86.04%   85.94%   -0.11%     
===========================================
  Files          439      439              
  Lines        58653    58653              
===========================================
- Hits         50469    50407      -62     
- Misses        8184     8246      +62     

Flag	Coverage Δ
rust	85.94% <ø> (-0.11%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/object-store/src/test_util.rs	0.00% <0.00%> (-100.00%)	⬇️
src/datanode/src/instance.rs	47.11% <0.00%> (-10.67%)	⬇️
tests-integration/src/test_util.rs	79.18% <0.00%> (-9.39%)	⬇️
src/servers/src/error.rs	38.59% <0.00%> (-5.27%)	⬇️
src/servers/src/opentsdb.rs	96.82% <0.00%> (-1.59%)	⬇️
src/common/substrait/src/df_expr.rs	30.16% <0.00%> (-0.66%)	⬇️
src/servers/src/postgres/handler.rs	82.84% <0.00%> (-0.37%)	⬇️
src/common/substrait/src/types.rs	51.92% <0.00%> (+0.96%)	⬆️
src/frontend/src/table/scan.rs	90.74% <0.00%> (+5.55%)	⬆️

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[killme2008]

LGTM