Fix autopkgtest failure

Autopkgtest fails due to the missing JAVA_SPECIFICATION_VERSION https://ci.debian.net/packages/j/jtreg7/testing/amd64/59633389/

Changes:

  * d/t/self-test: pass JAVA_SPECIFICATION_VERSION to make to fix
    autopkgtest regression.

Testing:

• autopkgtest:

$autopkgtest -U -B jtreg7_7.5.1+1+ds1-3_all.deb jtreg7/ -- schroot sid-amd64
...
if /bin/grep "headless: *true" /tmp/autopkgtest.iImTvl/build.BLV/real-tree/build/test/Basic.othervm/log > /dev/null ; then \
	if [ 21 -lt 18 ]; then \
		EXPECT_PASS=92 ; \
		EXPECT_FAIL=40 ; \
	else \
		EXPECT_PASS=89 ; \
		EXPECT_FAIL=28 ; \
	fi ; \
else \
	if [ 21 -lt 18 ]; then \
		EXPECT_PASS=94 ; \
		EXPECT_FAIL=44 ; \
	else \
		EXPECT_PASS=91 ; \
		EXPECT_FAIL=32 ; \
	fi ; \
fi ; \
echo "Expect: Test results: passed: ${EXPECT_PASS}; failed: ${EXPECT_FAIL}; error: 88" ; \
/bin/grep -s "Test results: passed: ${EXPECT_PASS}; failed: ${EXPECT_FAIL}; error: 88" /tmp/autopkgtest.iImTvl/build.BLV/real-tree/build/test/ReportOnlyTest.jt.log  > /dev/null
Expect: Test results: passed: 89; failed: 28; error: 88
echo /tmp/autopkgtest.iImTvl/build.BLV/real-tree/build/test/ReportOnlyTest.ok passed at `date` > /tmp/autopkgtest.iImTvl/build.BLV/real-tree/build/test/ReportOnlyTest.ok
echo /tmp/autopkgtest.iImTvl/build.BLV/real-tree/build/test/GroupTest.ok passed at `date` > /tmp/autopkgtest.iImTvl/build.BLV/real-tree/build/test/GroupTest.ok
echo /tmp/autopkgtest.iImTvl/build.BLV/real-tree/build/test/Group2Test.ok passed at `date` > /tmp/autopkgtest.iImTvl/build.BLV/real-tree/build/test/Group2Test.ok
/bin/grep -s 'Test results: error: 2' /tmp/autopkgtest.iImTvl/build.BLV/real-tree/build/test/threadCleanupTest/jt.log  > /dev/null
/bin/grep -s 'Problem cleaning up the following threads:' /tmp/autopkgtest.iImTvl/build.BLV/real-tree/build/test/threadCleanupTest/work/Test_id0.jtr
JavaTest Message: Problem cleaning up the following threads:
/bin/grep -s 'Problem cleaning up the following threads:' /tmp/autopkgtest.iImTvl/build.BLV/real-tree/build/test/threadCleanupTest/work/Test_id1.jtr
JavaTest Message: Problem cleaning up the following threads:
echo "test passed at `date`" > /tmp/autopkgtest.iImTvl/build.BLV/real-tree/build/test/threadCleanupTest.ok
All (224) selected tests completed successfully
Warning: JDK8HOME not set; some tests may not have been executed
Warning: JDK9HOME not set; some tests may not have been executed
Warning: JDK10HOME not set; some tests may not have been executed
Warning: JDK14HOME not set; some tests may not have been executed
Warning: JDK18HOME not set; some tests may not have been executed
Warning: HEADLESS is set; some tests may not have been executed
make: Leaving directory '/tmp/autopkgtest.iImTvl/build.BLV/real-tree/make'
autopkgtest [08:52:47]: test self-test: -----------------------]
autopkgtest [08:52:47]: test self-test:  - - - - - - - - - - results - - - - - - - - - -
self-test            PASS
autopkgtest [08:52:47]: @@@@@@@@@@@@@@@@@@@@ summary
self-test            PASS