Extend test coverage for SQL T621(Enhanced numeric functions)

Test coverage for SQL T621(Enhanced numeric functions) is poor.
Let's increase the test coverage. 

ref - test/sql/function/numeric/test_pg_math.test

T621 defines:
1. natural logarithm (supported, test: test_pg_math.test)
2. exponential function (supported, test: test_pg_math.test)
3. power function (supported, test: test_pow.test)
4. square root (supported, test: test_pg_math.test)
5. ceiling function (supported, test: test_pg_math.test)
6. STDDEV_POP, STDDEV_SAMP, VAR_POP, VAR_SAMP (not supported, test: test_stddev.test_ignore)
7. binary set function type, i.e :
COVAR_POP, COVAR_SAMP, CORR, REGR_SLOPE, REGR_INTERCEPT, REGR_COUNT, REGR_R2, REGR_AVGX, REGR_AVGY, REGR_SXX, REGR_SYY, REGR_SXY (not supported, test: test_covar.test_ignore)