Re: Document how to run only specific regress tests

hi.

maybe verbose, the following is the my intended changes based on your patch.
what do you think?

<sect2 id="regress-subset">
<title>Running Specific Tests</title>
<para>
You can run a subset of the regression tests through the <envar>TESTS</envar>
environment variable.
One way to do it is set <envar>TESTS</envar> environment variable beforehand:
<screen>
<userinput>export TESTS="test_setup boolean char"</userinput>
</screen>
You can also specify it while invoking the regress tests command,
one of example:
<screen>
make check-tests TESTS="test_setup boolean char"
</screen>
If the <envar>TESTS</envar> environment variable is set when the regression
tests are run, it overrides the original <envar>TESTS</envar> environment
variable.
</para>
<para>
All the regression tests are stored in <filename
class='directory'>src/test/regress/sql</filename>. Each
individual regression test
name is the corresponding file name omit the file name extension
(<literal>.sql</literal>). For example, the regression test name of
<literal>src/test/regress/sql/partition_join.sql</literal> is
<literal>partition_join</literal>. Tests specified by the
<envar>TESTS</envar>
environment variable will be run in a sequential order.
<literal>TESTS="test_setup copy' </literal> will first run
<literal>test_setup</literal> then <literal>copy</literal>. Note that
sometimes one test may depend on other tests, so you might get unexpected
failures if the dependent test is not specified. For example, to run
<literal>select_parallel</literal> test, to avoid failure, you can use
<screen>
make check-tests TESTS="test_setup create_misc create_index select_parallel"
</screen>
</para>
</sect2>