Add a test fixture

It only initializes QMK once, and clears the matrix after each test.

tests/test_common/matrix.c

@@ -22,7 +22,7 @@
 static matrix_row_t matrix[MATRIX_ROWS] = {};
 
 void matrix_init(void) {
-    memset(matrix, 0, sizeof(matrix));
+    clear_all_keys();
     matrix_init_quantum();
 }
 
@@ -54,3 +54,7 @@ void press_key(uint8_t col, uint8_t row) {
 void release_key(uint8_t col, uint8_t row) {
     matrix[row] &= ~(1 << col);
 }
+
+void clear_all_keys(void) {
+    memset(matrix, 0, sizeof(matrix));
+}