Only trigger autopkgtests for some key packages for gcc-*

Through the usual reverse dependency triggering, gcc-* usually triggers many
hundreds of (mostly universe) tests via libgccN. But:

 - This does not help to prevent compiler regressions: as all packages are
   built in -proposed anyway, the new compiler is being used immediately, so we
   can't hold it back in -proposed.

 - It does not trigger toolchain tests which actually are affected, most
   importantly binutils and linux.

 - This puts enormous stress onto our test infrastructure.

So special case gcc by triggering binutils and linux, and fglrx-installer as a
typical (and important) example of a DKMS package which also needs a compiler,
and libreoffice as our favourite tool chain stress test to cover libgccN.

autopkgtest.py

@@ -162,6 +162,20 @@ class AutoPackageTest(object):
 
         tests = []
 
+        # gcc-* triggers tons of tests via libgcc1, but this is mostly in vain:
+        # gcc already tests itself during build, and it is being used from
+        # -proposed, so holding it back on a dozen unrelated test failures
+        # serves no purpose. Just check some key packages which actually use
+        # gcc during the test, and libreoffice as an example for a libgcc user.
+        if src.startswith('gcc-'):
+            for test in ['binutils', 'fglrx-installer', 'libreoffice', 'linux']:
+                try:
+                    tests.append((test, self.britney.sources['testing'][test][VERSION]))
+                except KeyError:
+                    # no package in that series? *shrug*, then not (mostly for testing)
+                    pass
+            return tests
+
         srcinfo = sources_info[src]
         # we want to test the package itself, if it still has a test in
         # unstable

tests/test_autopkgtest.py

@@ -1137,6 +1137,19 @@ fancy 1 i386 linux-meta-lts-grumpy 1
             [('linux-generic', {'Source': 'linux-meta'}, None)],
             {'linux-meta': (False, {'lxc 1': {'amd64': 'RUNNING', 'i386': 'RUNNING'}})})
 
+    def test_gcc(self):
+        '''gcc only triggers some key packages'''
+
+        self.data.add('binutils', False, {}, testsuite='autopkgtest')
+        self.data.add('linux', False, {}, testsuite='autopkgtest')
+        self.data.add('notme', False, {'Depends': 'libgcc1'}, testsuite='autopkgtest')
+
+        exc = self.do_test(
+            [('libgcc1', {'Source': 'gcc-5', 'Version': '2'}, None)],
+            {'gcc-5': (False, {'binutils 1': {'amd64': 'RUNNING', 'i386': 'RUNNING'},
+                               'linux 1': {'amd64': 'RUNNING', 'i386': 'RUNNING'}})})[1]
+        self.assertNotIn('notme 1', exc['gcc-5']['tests']['autopkgtest'])
+
     def test_disable_adt(self):
         '''Run without autopkgtest requests'''