Avoid propagation of .mvn/maven.config to nested generated projects

When generating a project and testing it, we don't want the config
coming from .mvn/maven.config to interfere, now that we are using it to
pass build arguments to the root on CI.

See quarkusio@67f7030

test-framework/maven/src/main/java/io/quarkus/maven/it/MojoTestBase.java

@@ -74,6 +74,8 @@ public static File initEmptyProject(String name) {
         }
         boolean mkdirs = tc.mkdirs();
 
+        initDotMvn(tc);
+
         Logger.getLogger(MojoTestBase.class.getName())
                 .log(Level.FINE, "test-classes created? %s", mkdirs);
         return tc;
@@ -91,6 +93,9 @@ public static File initProject(String name) {
         if (!in.isDirectory()) {
             throw new RuntimeException("Cannot find directory: " + in.getAbsolutePath());
         }
+
+        initDotMvn(in);
+
         return in;
     }
 
@@ -118,6 +123,9 @@ public static File initProject(String name, String output) {
         boolean mkdirs = out.mkdirs();
         Logger.getLogger(MojoTestBase.class.getName())
                 .log(Level.FINE, out.getAbsolutePath() + " created? " + mkdirs);
+
+        initDotMvn(out);
+
         try {
             org.codehaus.plexus.util.FileUtils.copyDirectoryStructure(in, out);
         } catch (IOException e) {
@@ -127,6 +135,16 @@ public static File initProject(String name, String output) {
         return out;
     }
 
+    /**
+     * Initialize an empty .mvn to make sure we don't inherit from .mvn/maven.config from the root
+     * <p>
+     * .mvn/maven.config is used to pass args that might be too long for the Windows command line.
+     */
+    private static void initDotMvn(File projectDirectory) {
+        File dotMvn = new File(projectDirectory, ".mvn");
+        dotMvn.mkdirs();
+    }
+
     public static void filter(File input, Map<String, String> variables) throws IOException {
         DevModeClient.filter(input, variables);
     }