Add test to ensure search tabs behaviour

src/test/rustdoc-gui/search-tab-selection-if-current-is-empty.goml

@@ -0,0 +1,21 @@
+goto: file://|DOC_PATH|/index.html
+write: (".search-input", "Foo")
+// Waiting for the search results to appear...
+wait-for: "#titles"
+assert: ("#titles > button:nth-of-type(1)", "class", "selected")
+
+// To go back to the original "state"
+goto: file://|DOC_PATH|/index.html
+write: (".search-input", "-> String")
+// Waiting for the search results to appear...
+wait-for: "#titles"
+// With this search, only the last tab shouldn't be empty so it should be selected.
+assert: ("#titles > button:nth-of-type(3)", "class", "selected")
+
+// To go back to the original "state"
+goto: file://|DOC_PATH|/index.html
+write: (".search-input", "-> Something")
+// Waiting for the search results to appear...
+wait-for: "#titles"
+// With this search, all the tabs are empty so the first one should remain selected.
+assert: ("#titles > button:nth-of-type(1)", "class", "selected")