* Fix quirk of isolation2 test

- Fix the line continuation behaviour and document
Instead of matching "; *$" to detect the end of a sql end, the original
code was using ";$". This creates a totally different output for tests
like:
a)
SELECT * FROM t1;
SELECT * FROM t2;
b)
SELECT * FROM t1;<space>
SELECT * FROM t2;
For a), two SELECT statements will be sent separately.
For b), they will be sent together as
        "SELECT * FROM t1; SELECT * FROM t2;"

Spaces are playing too much magic here, some of them could be a bug in
the test itself. For example, the oom_query test should NOT pass since
it misses error output in the expected.

This change unifies the behaviours by matching with "; *$". So if a line
is end with ';', no matter spaces follows it or not, the statement will
be sent at once.

- Document the known issue of multi SQL in one line.

This issue was found during debugging pipeline failure of #8211
Authored-by: NChen Mulong <muchen@pivotal.io>