selftest: devtool: set BB_HASHSERVE_UPSTREAM when setting SSTATE_MIRROR

selftest: devtool: set BB_HASHSERVE_UPSTREAM when setting SSTATE_MIRROR

* with my build/conf/local.conf:
  SSTATE_DIR = "/OE/build/poky/build/sstate-cache"
  these devtool tests will first set own SSTATE_DIR and the original one set as SSTATE_MIRROR:

2023-03-11 11:51:46,837 - oe-selftest - INFO - test_devtool_update_recipe_append (devtool.DevtoolUpdateTests.test_devtool_update_recipe_append)
2023-03-11 11:51:46,846 - oe-selftest - DEBUG - Appending to: /OE/build/poky/build/build-st-2023-03-11-patch2/devtool.DevtoolUpdateTests.test_devtool_update_recipe_append/build-st/conf/selftest.inc
SSTATE_DIR = "/OE/build/poky/build/build-st-2023-03-11-patch2/devtool.DevtoolUpdateTests.test_devtool_update_recipe_append/build-st/sstate_devtool"
SSTATE_MIRRORS += "file://.* file:////OE/build/poky/build/sstate-cache/PATH"

* but that unfortunately leads to a warning from sanity.bbclass
  about SSTATE_MIRRORS without matching BB_HASHSERVE, because
  BB_HASHSERVE is set to "auto" by default

  these tests failing with:

2023-03-11 11:55:39,610 - oe-selftest - INFO - ======================================================================
2023-03-11 11:55:39,610 - oe-selftest - INFO - FAIL: test_devtool_update_recipe_append_git (devtool.DevtoolUpdateTests.test_devtool_update_recipe_append_git)
2023-03-11 11:55:39,610 - oe-selftest - INFO - ----------------------------------------------------------------------
2023-03-11 11:55:39,611 - oe-selftest - INFO - Traceback (most recent call last):
  File "/OE/build/poky/meta/lib/oeqa/selftest/cases/devtool.py", line 1118, in test_devtool_update_recipe_append_git
    self.assertNotIn('WARNING:', result.output)
AssertionError: 'WARNING:' unexpectedly found in 'NOTE: Starting bitbake server...\nWARNING: You are using a local hash equivalence server but have configured an sstate mirror. This will likely mean no sstate will match from the mirror. You may wish to disable the hash equivalence use (BB_HASHSERVE), or use a hash equivalence server alongside the sstate mirror.\nLoading cache...done.\nLoaded 0 entries from dependency cache.\nParsing recipes...done.\nParsing of 947 .bb files complete (0 cached, 947 parsed). 1764 targets, 52 skipped, 0 masked, 0 errors.\n\nSummary: There was 1 WARNING message.\nINFO: Updating SRCREV in recipe mtd-utils-selftest_git.bb\nNOTE: Writing append file /tmp/devtoolqa1m2lh02v/layer/recipes-devtools/mtd/mtd-utils-selftest_git.bbappend'
----------------------------------------------------------------------

* just setting BB_HASHSERVE to empty doesn't work, because then we
  would need to disable OEEquivHash as well as it fails with:

  ERROR: OEEquivHash requires BB_HASHSERVE to be set

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>