testsuite: fix a couple tests in t0100-modprobe.t

Problem: A couple of tests in t0100-modprobe.t use `libexecdir`
as the expected path to `modprobe.d`, but this should be `datadir`.
This works for the build tree since `libexecdir` == `datadir`, but
will fail when testing installed Flux. Additionally, use of `$(pwd)`
as a search path entry may not be failsafe, since there is a chance
a different path could be printed by modprobe, e.g. in the case of
a symlink.

Fix these issues by using `datadir` for expected paths to modprobe
TOML files, and an explicit fake path in place of `$(pwd)` when
matching exact paths in output.

Additionally, add more useful debug output to these tests.

Author: grondo

t/t0100-modprobe.t

@@ -545,20 +545,27 @@ test_expect_success 'modprobe fails if task raises exception' '
 	grep "next:.*test exception" output${seq}
 '
 test_expect_success 'modprobe: FLUX_MODPROBE_PATH works' '
-	FLUX_MODPROBE_PATH=$(pwd) \
+	FLUX_MODPROBE_PATH=/test/path \
 	  flux modprobe rc1 --dry-run --verbose >path.out 2>&1 &&
-	grep "checking $(pwd)" path.out &&
+	test_debug "echo libexecdir=$(flux config builtin libexecdir)" &&
+	test_debug "echo datadir=$(flux config builtin datadir)" &&
+	test_debug "grep checking path.out" &&
+	test_debug "echo =-=-=" &&
+	grep "checking /test/path/modprobe" path.out &&
 	test_must_fail \
-	  grep $(flux config builtin libexecdir)/modprobe/modprobe.d path.out &&
+	  grep $(flux config builtin datadir)/modprobe/modprobe.d path.out &&
 	test_must_fail \
 	  grep $(flux config builtin libexecdir)/modprobe/rc1.d path.out
 '
 test_expect_success 'modprobe: FLUX_MODPROBE_PATH_APPEND works' '
-	FLUX_MODPROBE_PATH_APPEND=$(pwd) \
+	FLUX_MODPROBE_PATH_APPEND=/test/path \
 	  flux modprobe rc1 --dry-run --verbose >path-append.out 2>&1 &&
-	test_debug "cat path-append.out" &&
-	grep "checking $(pwd)" path-append.out &&
-	grep $(flux config builtin libexecdir)/modprobe/modprobe.d path-append.out &&
+	test_debug "echo libexecdir=$(flux config builtin libexecdir)" &&
+	test_debug "echo datadir=$(flux config builtin datadir)" &&
+	test_debug "grep checking path-append.out" &&
+	test_debug "echo =-=-=" &&
+	grep "checking /test/path/modprobe" path-append.out &&
+	grep $(flux config builtin datadir)/modprobe/modprobe.d path-append.out &&
 	grep $(flux config builtin libexecdir)/modprobe/rc1.d path-append.out
 '
 test_expect_success 'modprobe: detects missing required modprobe.toml keys' '