tests: cleanup vlib/v/checker/tests/run ; add unused_variable_warning.vv

vlib/v/checker/tests/assign_expr_unresolved_variables_err_chain.out

@@ -0,0 +1,13 @@
+vlib/v/checker/tests/assign_expr_unresolved_variables_err_chain.vv:2:7: error: undefined variable `b` (used before declaration) 
+    1 | fn main() {
+    2 |     a := b
+      |          ^
+    3 |     b := c
+    4 |     c := a
+vlib/v/checker/tests/assign_expr_unresolved_variables_err_chain.vv:3:7: error: undefined variable `c` (used before declaration) 
+    1 | fn main() {
+    2 |     a := b
+    3 |     b := c
+      |          ^
+    4 |     c := a
+    5 | }

vlib/v/checker/tests/run/assign_expr_unresolved_variables_err_chain.run.out

@@ -1,13 +0,0 @@
-vlib/v/checker/tests/run/assign_expr_unresolved_variables_err_chain.vv:2:7: error: undefined variable `b` (used before declaration) 
-    1 | fn main() {
-    2 |     a := b
-      |          ^
-    3 |     b := c
-    4 |     c := a
-vlib/v/checker/tests/run/assign_expr_unresolved_variables_err_chain.vv:3:7: error: undefined variable `c` (used before declaration) 
-    1 | fn main() {
-    2 |     a := b
-    3 |     b := c
-      |          ^
-    4 |     c := a
-    5 | }

vlib/v/checker/tests/run/unused_variable_warning.run.out

@@ -0,0 +1,15 @@
+vlib/v/checker/tests/run/unused_variable_warning.vv:3:2: warning: unused variable: `a`
+    1 | // NB: this test should compile and run, but it also should produce a compiler warning.
+    2 | fn main() {
+    3 |     a := 1
+      |     ^
+    4 |     b := 2
+    5 |     println('hello')
+vlib/v/checker/tests/run/unused_variable_warning.vv:4:2: warning: unused variable: `b`
+    2 | fn main() {
+    3 |     a := 1
+    4 |     b := 2
+      |     ^
+    5 |     println('hello')
+    6 | }
+hello

vlib/v/checker/tests/run/unused_variable_warning.vv

@@ -0,0 +1,6 @@
+// NB: this test should compile and run, but it also should produce a compiler warning.
+fn main() {
+	a := 1
+	b := 2
+	println('hello')
+}