test: Add a 'make qcheck' target for quicker testing

At present tests are quite slow to run, over a minute on my machine. This
presents a considerable barrier to bisecting for failures.

The slowest tests are the filesystem ones and the buildman --fetch-arch
test. Add a new 'qcheck' target that skips these tests. This reduces test
time down to about 40 second, still too long, but bearable.

Signed-off-by: Simon Glass <sjg@chromium.org>

test/README

@@ -10,11 +10,15 @@ Running tests
 
 To run most tests on sandbox, type this:
 
-    test/run
+    make check
 
 in the U-Boot directory. Note that only the pytest suite is run using this
 command.
 
+Some tests take ages to run. To run just the quick ones, type this:
+
+    make qcheck
+
 
 Sandbox
 -------