patch 8.2.3061: testing the shell option is incomplete and spread out

Problem:    Testing the shell option is incomplete and spread out.
Solution:   Move shell tests to one file and increase coverage. (Yegappan
            Lakshmanan, closes #8464)
diff --git a/src/testdir/Make_all.mak b/src/testdir/Make_all.mak
index 108c528..6feeecd 100644
--- a/src/testdir/Make_all.mak
+++ b/src/testdir/Make_all.mak
@@ -242,6 +242,7 @@
 	test_selectmode \
 	test_set \
 	test_sha256 \
+	test_shell \
 	test_shift \
 	test_shortpathname \
 	test_signals \
@@ -471,6 +472,7 @@
 	test_search.res \
 	test_search_stat.res \
 	test_selectmode.res \
+	test_shell.res \
 	test_shortpathname.res \
 	test_signals.res \
 	test_signs.res \