[skip changelog] Allow running specific unit tests (arduino#466)

This also adds some documentation showing how this works.

Author: matthijskooijman

CONTRIBUTING.md

@@ -59,6 +59,26 @@ see following paragraph):
 ```shell
 task test-integration
 ```
+### Running only some tests
+By default, all tests from all go packages are run. To run only unit
+tests from one or more specific packages, you can set the TARGETS
+environment variable, e.g.:
+
+    TARGETS=./arduino/cores/packagemanager task test-unit
+
+Alternatively, to run only some specific test(s), you can specify a regex
+to match against the test function name:
+
+    TEST_REGEX='^TestTryBuild.*' task test-unit
+
+Both can be combined as well, typically to run only a specific test:
+
+    TEST_REGEX='^TestFindBoardWithFQBN$' TARGETS=./arduino/cores/packagemanager task test-unit
+
+For integration test, the same options are supported. Note that when not
+specified, `TEST_REGEX` defaults to "Integration" to select only
+integration tests, so if you specify a broader regex, this could cause
+non-integration tests to be run as well.
 
 ### Integration tests
 

Taskfile.yml

@@ -21,12 +21,12 @@ tasks:
   test-unit:
     desc: Run unit tests only
     cmds:
-      - go test -short {{ default "-v" .GOFLAGS }} -coverprofile=coverage_unit.txt {{ default .DEFAULT_TARGETS .TARGETS }} {{.TEST_LDFLAGS}}
+      - go test -short -run '{{ default ".*" .TEST_REGEX }}' {{ default "-v" .GOFLAGS }} -coverprofile=coverage_unit.txt {{ default .DEFAULT_TARGETS .TARGETS }} {{.TEST_LDFLAGS}}
 
   test-integration:
     desc: Run integration tests only
     cmds:
-      - go test -run Integration {{ default "-v" .GOFLAGS }} -coverprofile=coverage_integ.txt {{ default .DEFAULT_TARGETS .TARGETS }} {{.TEST_LDFLAGS}}
+      - go test -run '{{ default "Integration" .TEST_REGEX }}' {{ default "-v" .GOFLAGS }} -coverprofile=coverage_integ.txt {{ default .DEFAULT_TARGETS .TARGETS }} {{.TEST_LDFLAGS}}
       - pytest test
 
   test-legacy: