tests: Fix running schematests directly from topdir

Previously it failed like:

$ ./tests/domainschematest
./tests/domainschematest: line 4: ./test-lib.sh: No such file or directory

tests/capabilityschematest

 #!/bin/sh
 
-: ${srcdir=.}
+: ${srcdir=$(dirname $0)}
 . $srcdir/test-lib.sh
 . $abs_srcdir/schematestutils.sh
 

tests/domaincapsschematest

 #!/bin/sh
 
-: ${srcdir=.}
+: ${srcdir=$(dirname $0)}
 . $srcdir/test-lib.sh
 . $abs_srcdir/schematestutils.sh
 

tests/domainschematest

 #!/bin/sh
 
-: ${srcdir=.}
+: ${srcdir=$(dirname $0)}
 . $srcdir/test-lib.sh
 . $abs_srcdir/schematestutils.sh
 

tests/domainsnapshotschematest

 #!/bin/sh
 
-: ${srcdir=.}
+: ${srcdir=$(dirname $0)}
 . $srcdir/test-lib.sh
 . $abs_srcdir/schematestutils.sh
 

tests/interfaceschematest

 #!/bin/sh
 
-: ${srcdir=.}
+: ${srcdir=$(dirname $0)}
 . $srcdir/test-lib.sh
 . $abs_srcdir/schematestutils.sh
 

tests/networkschematest

 #!/bin/sh
 
-: ${srcdir=.}
+: ${srcdir=$(dirname $0)}
 . $srcdir/test-lib.sh
 . $abs_srcdir/schematestutils.sh
 

tests/nodedevschematest

 #!/bin/sh
 
-: ${srcdir=.}
+: ${srcdir=$(dirname $0)}
 . $srcdir/test-lib.sh
 . $abs_srcdir/schematestutils.sh
 

tests/nwfilterschematest

 #!/bin/sh
 
-: ${srcdir=.}
+: ${srcdir=$(dirname $0)}
 . $srcdir/test-lib.sh
 . $abs_srcdir/schematestutils.sh
 

tests/secretschematest

 #!/bin/sh
 
-: ${srcdir=.}
+: ${srcdir=$(dirname $0)}
 . $srcdir/test-lib.sh
 . $abs_srcdir/schematestutils.sh
 

tests/storagepoolschematest

 #!/bin/sh
 
-: ${srcdir=.}
+: ${srcdir=$(dirname $0)}
 . $srcdir/test-lib.sh
 . $abs_srcdir/schematestutils.sh
 

tests/storagevolschematest

 #!/bin/sh
 
-: ${srcdir=.}
+: ${srcdir=$(dirname $0)}
 . $srcdir/test-lib.sh
 . $abs_srcdir/schematestutils.sh
 

tests/test-lib.sh

@@ -18,10 +18,11 @@
 #
 # Based on an idea from GNU coreutils
 
-test -z "$abs_srcdir" && abs_srcdir=$(pwd)
-test -z "$abs_builddir" && abs_builddir=$(pwd)
-test -z "$abs_top_srcdir" && abs_top_srcdir=$(pwd)/..
-test -z "$abs_top_builddir" && abs_top_builddir=$(pwd)/..
+_scriptdir="$(realpath $(dirname $0))"
+test -z "$abs_srcdir" && abs_srcdir=$_scriptdir
+test -z "$abs_builddir" && abs_builddir=$_scriptdir
+test -z "$abs_top_srcdir" && abs_top_srcdir=$_scriptdir/..
+test -z "$abs_top_builddir" && abs_top_builddir=$_scriptdir/..
 test -z "$LC_ALL" && LC_ALL=C
 
 # Skip this test if the shell lacks support for functions.