t4034: use test_config/test_unconfig to set/unset git config variables

Instead of using construct such as:
    test_when_finished "git config --unset <key>"
    git config <key> <value>
uses
    test_config <key> <value>
The latter takes care of removing <key> at the end of the test.

Additionally, instead of
     git config <key> ""
or
     git config --unset <key>
uses
     test_unconfig <key>
The latter doesn't failed if <key> is not defined.

Signed-off-by: Yann Droneaud <ydroneaud@opteya.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t4034-diff-words.sh

@@ -230,7 +230,7 @@ test_expect_success '.gitattributes override config' '
 '
 
 test_expect_success 'setup: remove diff driver regex' '
-	test_might_fail git config --unset diff.testdriver.wordRegex
+	test_unconfig diff.testdriver.wordRegex
 '
 
 test_expect_success 'use configured regex' '
@@ -335,8 +335,7 @@ test_expect_success 'word-diff with diff.sbe' '
 
 	c
 	EOF
-	test_when_finished "git config --unset diff.suppress-blank-empty" &&
-	git config diff.suppress-blank-empty true &&
+	test_config diff.suppress-blank-empty true &&
 	word_diff --word-diff=plain
 '
 
@@ -368,7 +367,7 @@ test_expect_success 'setup history with two files' '
 
 test_expect_success 'wordRegex for the first file does not apply to the second' '
 	echo "*.tex diff=tex" >.gitattributes &&
-	git config diff.tex.wordRegex "[a-z]+|." &&
+	test_config diff.tex.wordRegex "[a-z]+|." &&
 	cat >expect <<-\EOF &&
 		diff --git a/a.tex b/a.tex
 		--- a/a.tex