test: Declare thread usage of tests, to fix flakiness and speedup

test/unit/caching/caching_test.bzl

@@ -65,7 +65,10 @@ def caching_test(
             str(expected_action_count),
         ],
         local = True,
-        tags = ["unit"] + tags,
+        # Each test runs a nested Bazel invocation, so it needs a second
+        # thread. Declaring 2 CPUs makes Bazel reserve the capacity instead of
+        # overpacking the machine.
+        tags = ["unit", "cpu:2"] + tags,
         size = size,
         **kwargs
     )

test/unit/external_repository/external_test.bzl

@@ -97,6 +97,9 @@ def external_test(
         # No other way to ensure integrated bazel can find the repository
         local = True,
         size = size,
-        tags = tags,
+        # Each test runs a nested Bazel invocation, so it needs a second
+        # thread. Declaring 2 CPUs makes Bazel reserve the capacity instead of
+        # overpacking the machine.
+        tags = tags + ["cpu:2"],
         **kwargs
     )

test/unit/parse/BUILD

@@ -113,7 +113,13 @@ py_test(
     data = [":report_files"],
     local = True,
     main = "store_check.py",
-    tags = ["unit"],
+    tags = [
+        # The store test runs a CodeChecker server alongside the store client.
+        # Running it alongside other tests often make this test fail
+        # due to the codechecker server timing out.
+        "cpu:2",
+        "unit",
+    ],
     deps = [":helpers"],
 )
 
@@ -153,6 +159,12 @@ py_test(
     data = [":per_file_report_files"],
     local = True,
     main = "store_check.py",
-    tags = ["unit"],
+    tags = [
+        # The store test runs a CodeChecker server alongside the store client.
+        # Running it alongside other tests often make this test fail
+        # due to the codechecker server timing out.
+        "cpu:2",
+        "unit",
+    ],
     deps = [":helpers"],
 )