Clarify running unit tests documentation

Clarify that the test target must be built before attempting to run the
unit tests themsleves. The previous instruction only followed the chrome
build instructions. It was not immediately obvious that build chrome
would not also generate the unit_tests binary.

Change-Id: I2894d40926d6546a929ccfcdff4b08bc8f87f9f4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3428904
Reviewed-by: Roman Arora <romanarora@google.com>
Commit-Queue: Fred Shih <ffred@google.com>
Cr-Commit-Position: refs/heads/main@{#966576}

docs/linux/build_instructions.md

@@ -268,8 +268,14 @@ $ out/Default/chrome
 
 ## Running test targets
 
-You can run the tests in the same way. You can also limit which tests are
-run using the `--gtest_filter` arg, e.g.:
+First build the unit_tests binary by running the command:
+
+```shell
+$ autoninja -C out/Default unit_tests
+```
+
+You can run the tests by running the unit_tests binary. You can also limit which
+tests are run using the `--gtest_filter` arg, e.g.:
 
 ```shell
 $ out/Default/unit_tests --gtest_filter="PushClientTest.*"