tests: add missing double quotes to included library paths

Fix inclusion errors which would occur if the $TEST_DIRECTORY had $IFS
whitespace in it.

See d42bab4 (core.fsyncmethod: tests for batch mode, 2022-04-04)
and a242c15 (vimdiff: integrate layout tests in the unit tests
framework ('t' folder), 2022-03-30) for the two relevant commits. Both
were first released with v2.37.0-rc0 (and were also part of v2.37.0).

Signed-off-by: Ævar Arnfjörð Bjarmason <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: avar

t/t3700-add.sh

@@ -8,7 +8,7 @@ test_description='Test of git add, including the -- option.'
 TEST_PASSES_SANITIZE_LEAK=true
 . ./test-lib.sh
 
-. $TEST_DIRECTORY/lib-unique-files.sh
+. "$TEST_DIRECTORY"/lib-unique-files.sh
 
 # Test the file mode "$1" of the file "$2" in the index.
 test_mode_in_index () {

t/t3903-stash.sh

@@ -9,7 +9,7 @@ GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME=main
 export GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME
 
 . ./test-lib.sh
-. $TEST_DIRECTORY/lib-unique-files.sh
+. "$TEST_DIRECTORY"/lib-unique-files.sh
 
 test_expect_success 'usage on cmd and subcommand invalid option' '
 	test_expect_code 129 git stash --invalid-option 2>usage &&

t/t7609-mergetool--lib.sh

@@ -7,7 +7,7 @@ Testing basic merge tools options'
 . ./test-lib.sh
 
 test_expect_success 'mergetool --tool=vimdiff creates the expected layout' '
-	. $GIT_BUILD_DIR/mergetools/vimdiff &&
+	. "$GIT_BUILD_DIR"/mergetools/vimdiff &&
 	run_unit_tests
 '