Update perf_profiler_bpf_test to use UnitConnector. (#1283)

Summary: we convert `perf_profiler_bpf_test` to use `UnitConnector` to
share code across different tests and binaries.

We also refactor the Java test case to separately test the expectation
that Java symbolization artifacts are removed after processes stop. The
main Java test case then conforms to the pattern established for Go and
cpp.

We also update some of the helper functions in the test to tidy up the
common test cases.

Type of change: /kind cleanup

Test Plan: existing tests & runs per test.
- [x] `bazel test //src/stirling/source_connectors/perf_profiler/...
--config bpf --runs_per_test 16` ([all tests
passed](https://phab.corp.pixielabs.ai/P372)).

---------

Signed-off-by: Pete Stevenson <jps@pixielabs.ai>

Author: Pete Stevenson

src/stirling/core/unit_connector.h

@@ -111,6 +111,22 @@ class UnitConnector {
     return Status::OK();
   }
 
+  Status ResetConnectorContext(absl::flat_hash_set<md::UPID> upids) {
+    // Stop transferring data.
+    PX_RETURN_IF_ERROR(StopTransferDataThread());
+    if (upids.size() == 0) {
+      // The upids set is empty: trace all processes and use automatic context refresh.
+      ctx_ = std::make_unique<EverythingLocalContext>();
+    } else {
+      // The upids set is non-empty: we will trace only processes identified by that set.
+      ctx_ = std::make_unique<StandaloneContext>(upids);
+    }
+    // Restart.
+    PX_RETURN_IF_ERROR(StartTransferDataThread());
+
+    return Status::OK();
+  }
+
   Status Init(absl::flat_hash_set<md::UPID> upids = {}) {
     if (upids.size() == 0) {
       // Enter this branch if Init() is called with no arguments (i.e. with a default empty set).

src/stirling/source_connectors/perf_profiler/BUILD.bazel

@@ -51,6 +51,7 @@ pl_cc_bpf_test(
         "//src/stirling/source_connectors/perf_profiler/testing/cc:profiler_test_app_fib",
         "//src/stirling/source_connectors/perf_profiler/testing/go:profiler_test_app_sqrt_go",
         "//src/stirling/source_connectors/perf_profiler/testing/java:graal-vm-aot-test",
+        "//src/stirling/source_connectors/perf_profiler/testing/java:profiler_test",
     ],
     defines = ['JDK_IMAGE_NAMES=\\"%s\\"' % ",".join(jdk_image_names)],
     env = {"QEMU_CPU_COUNT": "4"},