gh-108418: Speed up bigmem compression tests in dry mode (GH-108419)

Only generate and compress small amount of random data in dry run.

Lib/test/test_bz2.py

@@ -721,10 +721,10 @@ class BZ2DecompressorTest(BaseTest):
     @bigmemtest(size=_4G + 100, memuse=3.3)
     def testDecompress4G(self, size):
         # "Test BZ2Decompressor.decompress() with >4GiB input"
-        blocksize = 10 * 1024 * 1024
+        blocksize = min(10 * 1024 * 1024, size)
         block = random.randbytes(blocksize)
         try:
-            data = block * (size // blocksize + 1)
+            data = block * ((size-1) // blocksize + 1)
             compressed = bz2.compress(data)
             bz2d = BZ2Decompressor()
             decompressed = bz2d.decompress(compressed)

Lib/test/test_lzma.py

@@ -352,10 +352,10 @@ class CompressorDecompressorTestCase(unittest.TestCase):
     @bigmemtest(size=_4G + 100, memuse=3)
     def test_decompressor_bigmem(self, size):
         lzd = LZMADecompressor()
-        blocksize = 10 * 1024 * 1024
+        blocksize = min(10 * 1024 * 1024, size)
         block = random.randbytes(blocksize)
         try:
-            input = block * (size // blocksize + 1)
+            input = block * ((size-1) // blocksize + 1)
             cdata = lzma.compress(input)
             ddata = lzd.decompress(cdata)
             self.assertEqual(ddata, input)

Lib/test/test_zlib.py

@@ -989,10 +989,10 @@ class ZlibDecompressorTest(unittest.TestCase):
     @bigmemtest(size=_4G + 100, memuse=3.3)
     def testDecompress4G(self, size):
         # "Test zlib._ZlibDecompressor.decompress() with >4GiB input"
-        blocksize = 10 * 1024 * 1024
+        blocksize = min(10 * 1024 * 1024, size)
         block = random.randbytes(blocksize)
         try:
-            data = block * (size // blocksize + 1)
+            data = block * ((size-1) // blocksize + 1)
             compressed = zlib.compress(data)
             zlibd = zlib._ZlibDecompressor()
             decompressed = zlibd.decompress(compressed)