t: don't redefine test_config() in various places

test_config() is already a well-defined function in
test-lib-functions.sh.  Don't duplicate it unnecessarily in:

  t4018-diff-funcname.sh
  t7810-grep.sh
  t7811-grep-open.sh

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
t3200 (branch): modernize style

Style is inconsistent throughout the file.  Make the following
changes:

1. Indent everything with tabs.

2. Put the opening quote (') for the test in the same line as
   test_expect_succcess, and the closing quote on a line by itself.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>