make: targeted packages

This commit lets us perform make commands of the following form:
`make unit pkg=accounts` which will run all the tests in the specified
package. Or you can do `make unit pkg=accounts case=TestAccountStore` to
run a specific test in the specified package.

make/testing_flags.mk

@@ -12,12 +12,35 @@ endif
 # If a specific unit test case is being target, construct test.run filter.
 ifneq ($(case),)
 TEST_FLAGS += -test.run=$(case)
+UNIT_TARGETED = yes
+endif
+
+# If specific package is being unit tested, construct the full name of the
+# subpackage.
+ifneq ($(pkg),)
+UNITPKG := $(PKG)/$(pkg)
+COVER_PKG := $(PKG)/$(pkg)
+UNIT_TARGETED = yes
+GOLIST = echo '$(PKG)/$(pkg)'
 endif
 
 # Add any additional tags that are passed in to make.
 ifneq ($(tags),)
 DEV_TAGS += ${tags}
 endif
 
+# UNIT_TARGETED is undefined iff a specific package and/or unit test case is
+# not being targeted.
+UNIT_TARGETED ?= no
+
+# If a specific package/test case was requested, run the unit test for the
+# targeted case. Otherwise, default to running all tests.
+ifeq ($(UNIT_TARGETED), yes)
+UNIT := $(GOTEST) -tags="$(DEV_TAGS) $(COMPILE_TAGS)" $(TEST_FLAGS) $(UNITPKG)
+UNIT_RACE := $(GOTEST) -tags="$(DEV_TAGS) $(COMPILE_TAGS)" $(TEST_FLAGS) -race $(UNITPKG)
+endif
+
+ifeq ($(UNIT_TARGETED), no)
 UNIT := $(GOLIST) | $(XARGS) env $(GOTEST) -tags="$(DEV_TAGS) $(COMPILE_TAGS)" $(TEST_FLAGS)
-UNIT_RACE := $(UNIT) -race
+UNIT_RACE := $(UNIT) -race
+endif