Add bats unit testing to repository

Will recursively look for `*.bats` files and run them with bats-core test suite. Simple sample unit tests for existing scripts are included in `.config/shell/rc.d/test`.

.config/shell/rc.d/test/env-wiki-root.bats

@@ -0,0 +1,23 @@
+setup() {
+  testfile="$BATS_TEST_DIRNAME/../env-wiki-root.sh"
+}
+
+teardown() {
+  export WIKIROOT=""
+}
+
+@test "Populates WIKIROOT environment variable" {
+  source $testfile
+  [ -n "$WIKIROOT" ]
+}
+
+@test "Sets WIKIROOT to correct default path" {
+  source $testfile
+  [ "$WIKIROOT" = "$HOME/Nextcloud/Notes/" ]
+}
+
+@test "Leaves WIKIROOT as is if it is already set" {
+  export WIKIROOT="MY/CUSTOM/DIR"
+  source $testfile
+  [ "$WIKIROOT" = "MY/CUSTOM/DIR" ]
+}