core tests: add custom string matchers

jadx-core/src/test/java/jadx/tests/utils/CountString.java

+package jadx.tests.utils;
+
+import org.hamcrest.core.SubstringMatcher;
+
+public class CountString extends SubstringMatcher {
+
+	private final int count;
+
+	public CountString(int count, String substring) {
+		super(substring);
+		this.count = count;
+	}
+
+	@Override
+	protected boolean evalSubstringOf(String string) {
+		return this.count == countStr(string, substring);
+	}
+
+	@Override
+	protected String relationship() {
+		return "containing " + count + " occurrence of";
+	}
+
+	private static int countStr(String string, String substring) {
+		int cnt = 0;
+		int idx = 0;
+		while ((idx = string.indexOf(substring, idx)) != -1) {
+			idx++;
+			cnt++;
+		}
+		return cnt;
+	}
+}

jadx-core/src/test/java/jadx/tests/utils/JadxMatchers.java

+package jadx.tests.utils;
+
+import org.hamcrest.Matcher;
+
+public class JadxMatchers {
+
+	public static Matcher<String> countString(int count, String substring) {
+		return new CountString(count, substring);
+	}
+
+	public static Matcher<String> containsOne(String substring) {
+		return new CountString(1, substring);
+	}
+}